BHL holding forth is a familiar sight in France. Bernard-Henri Lévy — the plutocrat philosopher always known here by his initials — has practically lived on stage for 40 years, generally on French TV. His immaculate white shirt open almost to the navel, a stream of quotes from dead philosophers dropping from his tongue, he opines with unshakeable confidence on any topic, ones he knows a lot about and ones he doesn’t. Today, over tea in a patrician Parisian hotel library, he’s talking about Brexit.

Born in Algeria in 1948, the son of a Jewish timber merchant who became a multimillionaire, BHL was an Anglophile long before he became a celebrity intellectual. He says, “My generation was told as a child, ‘You exist thanks to Churchill and the pilots of the Royal Air Force.’ In families like mine, the link between France and England was a link of blood. All my adolescence I spent at least half the summer in England. I was two summers in Crawley, Sussex, of which I have marvellous memories.”

BHL considers himself more than a mere thinker. Like his British literary heroes Lord Byron and TE Lawrence, he is a man of action who aims to change the world. On June 4, backed by the Hexagon Society, a London-based French cultural charity, he will stage his one-man play Last Exit Before Brexit at London’s Cadogan Hall.

The play is a plea to Britain — or Angleterre (“England”), as BHL usually calls it — to realise that it’s the intellectual heart of Europe and should therefore stay in and help reform the “soulless” European Union. He says, “There are no causes today that mobilise me as much as this one: to convince the English that Brexit would be an unimaginable regression of civilisation.” He describes himself as “truly a liberal” (a rare breed in France) and he identifies London as the capital of liberal Europe. The Europe that treats markets and democracy as intrinsically linked, he explains, “was born in 1945 in the few streets between Churchill’s war cabinet and the City [of London]. The software of Europe is English. Liberal Europe is not a French Europe. The Europe of today’s liberal democratic open societies is Keynes, Adam Smith, Karl Popper.”

He doesn’t yet have a script of the play to show me, but from his description it takes the form of a nearly two-hour stream-of-consciousness monologue by an imaginary French writer who has to give the keynote speech at a conference on Brexit. “The suspense of the play”, explains BHL, “is that he cannot manage to write the speech.”

Mocking BHL is easy. In fact, it’s a French national pastime. Still, he knows his European thought, and somewhere in his play is a crucial contemporary topic: Brexit and the Franco-British relationship.

Most French people switched off from Brexit long ago, but a small coterie of Parisian civil servants, businesspeople and soldiers is still thinking hard about it every day. I interviewed many of its members (including numerous officials who cannot be named) and found that BHL has indeed hit upon the subjects that most concern them: “Frexit”, Europe’s security and the London-Paris rivalry. It’s just that whereas BHL hopes to save Britain from the cliff-edge, the French Brexit community is intent on shaping a Brexit that works for France. In interviews with French officials, I noticed that mentions of Brexit often produced a smirk.

In Britain there is a presumption, especially among Brexiters, that France wants to “punish” the UK for Brexit. They note that France has been even more rigid than most other EU member states in the Brexit negotiations. President Emmanuel Macron has said that if the UK leaves the EU’s structures, it will lose all the benefits. “There should be no cherry-picking in the single market,” he says. A British diplomat sighs: “The French are being very French.”

However, it’s misguidedly Anglocentric to interpret French policy as anti-British. Rather, French policy is pro-French. More than perhaps any other major western country today, France has a clear idea of its long-term national interest — which, unlike Germany, it’s never abashed to pursue. London still doesn’t know what it wants from Brexit, but Paris does. When French civil servants began planning for Brexit two years ago, they were told to rethink Franco-British relations from scratch, as if the UK was just another “third country”.

France starts from a friendly attitude. BHL’s Anglophilia is common among Macron’s generation of the French elite. Remarkably for two ambitious military powers separated by just 20 miles of sea, “We haven’t fought each other for 200 years,” notes Peter Ricketts, who was British ambassador in Paris until January 2016. “The French would have much preferred it if Brexit hadn’t happened,” Ricketts adds. But now that it has, he warns, they will be hard-nosed: “There is no nostalgia in the French.”

In chronological order, the first favour Brexit can do for the French ruling class is help stamp out the fantasy of Frexit. When I mentioned to one French official that the French in 2016 might also have voted to leave the EU, he replied, “Yes. But we wouldn’t have been dumb enough to hold a referendum.”

Just last spring, Marine Le Pen reached the second round of the French presidential elections promising to hold one. But since Brexit hit trouble, the leader of the far-right Front National has gone quiet on Frexit. Now she claims she can improve French life “without leaving either Europe or the euro”.

But Frexit could come back from the dead. BHL sees little difference between French and British nationalism: “In both cases there’s the fantasy of a return to a lost identity.” He warns that compared with previous anti-democratic movements such as Marxism, Nazism and Islamic fundamentalism, “Populism may be the one that works best, that’s most convincing. Macron is fragile.”

Few in Britain will even notice the European parliamentary elections of May 2019, but they loom large on Macron’s calendar. He aims to beat the FN, and win a moral mandate for the rest of his five-year term. To make sure Frexit stays dead, Brexit has to be costly. Britain can’t have back doors into the single market. If it leaves, France wants to make sure it’s entirely out.

That would also assuage France’s biggest fear for post-Brexit Britain: that the UK sets itself up as a low-regulation zone on Europe’s doorstep. British officials keep assuring the French that this isn’t the plan. The French largely believe this, but they ask: what does the UK do in five years, if Brexit goes badly? Then slashing regulations on everything from food to environmental to worker standards might prove irresistible. French companies have lobbied Brussels to ensure this doesn’t happen, says Georgina Wright of London think-tank Chatham House.

The second thing the French ruling class wants from Brexit is to make Paris as great as London. Market-friendly London has become a capital of globalisation. But now, says French economist and urbanist Robin Rivaton, French elite members of Macron’s generation “have a desire for revenge on London. We have only known London triumphant, which has crushed Paris.”

If anyone can turn Paris into the new capital of liberal Europe, it’s France’s most liberal president. Already, he has liberalised France’s labour markets and cut taxes. BHL sees a kindred spirit: “Macron, like me — though he’s younger — couldn’t exist without English ideology. He’s an Englishman of France.” In fact, Macron could have been a Londoner. He had agreed to become a visiting fellow at the London School of Economics when in 2014 he was appointed French economy minister.

So far, most estimates of Brexit’s impact on Paris are modest. Only about 2,280 jobs in finance will move from London to Paris or be created there because of Brexit, a Reuters survey estimated in March. But Ross McInnes, a Frenchman of Australian origin who chairs French aerospace and defence group Safran, and is “economic ambassador” for Greater Paris, says: “It’s many more than that. The management of bigger businesses, the last thing they want to do is create uncertainty among their population in London. You don’t walk into a trading room and say, ‘Right, we’re off to Paris or Frankfurt or Amsterdam in a year’s time,’ without telling people who’s coming. You have to come to your employees with a plan, and no loose ends. That’s why you don’t see much about it in the headlines.

“What I see is major US and non-European institutions actually taking office space. In one or two cases we’ve had to intervene and help them with the City of Paris in terms of getting planning permission speedily. If you divide the number of square feet that people are looking at by the typical occupancy rate in the financial sector, these are individual movements of 500 to 700 people.” How big a total shift to Paris does he expect? “I wouldn’t call it massive in the scope of 350,000 finance jobs in the City. ‘Significant’ would probably do it.”

McInnes expects more announcements of moves in early 2019: “People start looking at schooling for September 2019.” Even by then, the final status of Brexit may still be unclear: “Businesses don’t like uncertainty. But the UK government has done nothing to reduce that uncertainty.” British officials grumble that France has deliberately prolonged uncertainty in the Brexit talks, to encourage companies to move. McInnes says, “None of us wanted Brexit to happen. But we’re consistent. So on the financial passport, well no, you don’t get to keep it.”

Paris’s budding tech sector aims to poach from London too. Xavier Niel, the French billionaire tech entrepreneur who backs the Parisian start-up hub Station F (where the working language is English), told Paris’s Anglo-American Press Association: “I don’t think any French people are going to London now to create their start-up. Maybe a few, but it would be an original choice. There is British instability, Brexit, then maybe Corbyn. I don’t know much about Corbyn but I meet young people who tell me, ‘We’re going to have a communist in England. We will become France.’ ” Under Corbyn, liberal Europe might need a new capital.

Macron is especially hopeful of luring French expats back from a post-Brexit London. Catherine Fieschi, French head of the London-based think-tank Counterpoint, who attended some of his En Marche party’s campaign meetings in London last year, says: “It was like an appeal to people in exile. People were saying, ‘We’re going to be able to go home again! The homeland!’

Brexit won’t destroy London’s centuries-old business infrastructure. Nor would Paris want that: the two capitals are rivals, but in some ways they also operate as a single megacity. They need each other. Still, house prices already reveal a rebalancing. In the decade before the referendum, despite the financial crisis, the average London house price rose 86 per cent, according to the Office for National Statistics. Since then it has fallen in real terms. Sterling’s post-Brexit fall has further reduced the price difference with Paris: in euros, the average London house now costs just €533,344, or about €75,000 less than on the day before the Brexit vote. Meanwhile, Parisian prices have jumped under Macron, after years of stagnation. The Olympics provide a more symbolic indicator of the reversal: London beat Paris to host the 2012 Games. Now Paris is preparing to host in 2024.

Franco-British economic rivalry is real, and yet for French policymakers it’s almost a sideshow. What concerns them more is the scary outside world that has taken shape just in the two years since the Brexit referendum. Europe now feels squeezed by Vladimir Putin and abandoned by Donald Trump. While French and Brits joust over jobs and business, the issue of security is pushing them closer together than they have been in decades.

When Trump took office, Macron did what a British prime minister would typically do: position himself as the president’s best friend. At first his charm seemed to work. True, Trump abandoned the Paris climate accords anyway, but Macron felt that Trump at least listened to him.

That illusion has faded. This spring, Trump has gone rogue: he has imposed trade sanctions, pulled out of the Iran nuclear deal, and moved the US’s embassy in Israel to Jerusalem — in each case, against Macron’s wishes. Meanwhile US officials are telling Paris (not very nicely) to spend more in Afghanistan.

All this is pushing France into the UK’s arms. This spring, the neighbours have co-operated on joint strikes against Syria (with the US), a joint response to Russia’s alleged poisoning of the ex-spy Sergei Skripal in Salisbury, and joint protests to the US over the Iran deal. Europe’s two biggest intelligence services also constantly exchange information on potential terrorists.

Now Macron is mooting a new European Intervention Initiative. The EII — which could be launched next month — would be a coalition of the willing, for countries inside and outside the EU to join ad hoc military actions. (Brexiters needn’t worry about an EU army ever emerging: the pacifist Germans have defanged even a very modest French push for integration.) The EII has potential to become a mostly Franco-British vehicle. London likes it in principle, says Hans Kundnani of Chatham House. Already British brigadier Nick Nottingham is deputy commander of a French army division; French brigadier Hervé Bizeul has the same role in the British army’s First Division. It’s imaginable that one day the two armies could share bases abroad, a French official suggests.

In fact, the French say their main worry is that the UK is currently too weak and poor to do enough with them on defence. Only after much arm-twisting did the British send three RAF Chinook helicopters to Mali in January.

BHL’s earliest associations with the UK were of military might. “Britain won the war alone,” he says. “Well, with America,” and he rhapsodises about “the firmness of the souls of the English beneath the bombs”. He has always been a muscular philosopher. In 2011, he famously rang his old friend Nicolas Sarkozy, then French president, and persuaded him to intervene alongside Britain to oust Colonel Gaddafi in Libya. (BHL argues, contrary to popular opinion, that “The intervention couldn’t have gone better than it has. One hopes Libya will be better one day.”)

When I suggest that Brexit is more farce than tragedy, BHL disagrees. He thinks it could end very badly. “Who knows where this butterfly effect can lead? There is certainly a puppeteer called Putin. He has one strategic objective: the dislocation of Europe. It’s the story of his life.” But Putin and the Brexiters haven’t reckoned with one doughty French philosopher.