What happens if Britain walks away from the Brexit negotiations? Will it be like cutting the Gordian knot, allowing us to break free of EU tyranny, or will it result in the slow strangulation of the UK economy and a nightmare for citizens caught on the wrong side of the line? The short drop or the long drop?

Saturday’s ritualistic summit in Brussels, at which the EU 27 in effect signed a treaty of mutual assurance in their own blood does not bode well for the negotiations due to open with Britain sometime in June. It was as if their script had been written by Don Corleone, making us an offer we cannot refuse. But there is no need for us to panic. We shouldn’t listen to siren voices on the Right urging us to cut and run. The choice is not between Agincourt or Dunkirk. It is called diplomacy and we used to be rather good at it.

To begin, we can, I think, presume that the Prime Minister (Theresa May, that is, not Jeremy Corbyn) will choose to remain in her place throughout Day One. To leave the stage just as the curtain goes up would certainly be dramatic but would not produce the sort of reviews her British followers are hoping for. Fortunately, we are told that Mrs May prefers to calculate her every move, and such precipitate action would be entirely out of character. That said, if the 27 continue to act as if they are a gang out to teach Britain a lesson and if hardline elements within the Tory Party, backed by sections of the commentariat, get their way, a walkout on Week Two or Week Three cannot be excluded. 

So then what would happen? Let me lay it out for you. 

The European Commission would sue for alimony, taking its case to the International Court in The Hague. The process would be long and drawn out, during which everything else would be put on hold. It is likely, though far from certain, that the judgment would end in compromise. The £50 billion sought could be reduced to £40 billion, or even £30 billion – an outcome equally available, and just as likely, through direct face-to-face talks. But should our erstwhile partners feel they have been left seriously out of pocket by a welcher refusing to pay her debts, they will seek to extract their pound of flesh in other ways and the result will not be pretty. 

Were the PM then to announce that, what the Hell, Brexit means Brexit and no deal is the best deal available, the bad would quickly get worse. The full weight of WTO third-country tariffs would be imposed on imports from the UK. Britain would reciprocate, leading to a standoff that the 27 are in a better position to withstand long-term than the 1. The date for the opening of trade talks between London and Brussels would be officially set at Doomsday. City of London banks and other financial institutions would be informed that their passport into the Eurozone had been revoked, with immediate effect. The certainty of health cover and social provision would be withdrawn from British citizens living in the EU while, on the English side of the Channel, more ships and extra flights would be needed to convey Poles, Romanians, Spanish, French and others fleeing to their homelands, depriving the NHS, manufacturing industry, agriculture and the City of London of millions of their most valued employees. 

The fallout would be devastating. Old friendships and understandings would be sundered. Gibraltar would be up for grabs, Scottish independence and Irish unity would race up the political agenda and the border between France and the UK would be moved back to Dover, leading to the creation of a Jungle in the Garden of England.

You don’t believe any of this? Then you haven’t been paying attention. 

But we shouldn’t get ahead of ourselves. In the event of a standoff over the divorce settlement, it is not beyond the bounds of possibility that the two sides would at least cut a deal on citizens’ rights before bringing the shutters down. In that case, “only” trade, the debt, political cooperation and goodwill  would be left in limbo. But on the cash front, things could get ugly quickly. Older readers will recall how Mrs Thatcher demanded “her” money back in 1985, refusing to cooperate on anything until Britain won its rebate on overpayments into the EU budget. That is what it would be like after a Whitehall walkout, but with knobs on and the boot on the other foot.

Even if the outcome I have described turned out to be somewhat less lurid, it would in no sense be a vote-winner. The Falklands factor on which Thatcher built her reputation required a victory at sea, not a refusal to engage. Mrs May could find that, with inflation rising, sterling on the floor and members of the international community starting to think of Britain as a country left behind by history, like Spain and Portugal in the eighteenth century, the next general election, set for 2022, would hand the reins of power straight back to the Labour Party and the SNP. And God knows what would happen then. 

It would be madness. Only if the Prime Minister was Jack Sparrow and Britain was the Black Pearl could such a buccaneering approach to diplomacy even hope to succeed. Dear readers, I know Jack Sparrow. I’ve seen his movies. Mrs May is no Jack Sparrow.

Irresponsible hardliners should take note.