What if the UK opts for Brexit? What happens next?

The Prime Minister runs through his script, expressing sadness and disappointment. The words at times appear to catch in the Tory leader’s throat and then, as he accepts the blame for failing to persuade the British people to stay in the EU, he announces his decision to resign. There is a pause as the word hangs in the air.

Mr Cameron, his authority shattered by the referendum result, explains that he will stay on in Downing Street until his party can decide on a replacement. He nods, bites his lip, turns and disappears from view.

The next 24 hours are frantic. The markets are in turmoil, the pound slides, European leaders express dismay. The political kaleidoscope has been shaken.

Remainers swiftly begin the post mortem laced with recrimination and rancour. Labour MPs turn on Jeremy Corbyn, expressing disbelief and anger that their leader did too little too late to persuade traditional working class voters to reject Ukip’s demonisation of the EU. Talk of a leadership challenge is revived.

In Scotland, voices, Nationalist ones in particular, begin to insist that Nicola Sturgeon must call a second independence referendum. The Scots had voted 59 per cent to 41 per cent to stay in the EU.

The First Minister issues a holding statement and spends the weekend ensconced in Bute House, taking phonecalls. Some colleagues, most notably Alex Salmond, urge her to demand a second poll soon, others like Angus Robertson stress how public opinion is still too divided and volatile; to go too early could risk defeat; waiting until the dust settles is a far wiser course.

Westminster, meantime, is recalled to hear what the UK Government is going to do amid the markets mayhem. Mr Cameron spends his most uncomfortable time at the despatch box, fending off MPs’ questions, deflecting the schadenfreude of his eurosceptic colleagues and urging people to stay calm.

He reverts back to his pre-catastrophe stage before he spoke of Brexit being a bomb under the economy and of it sparking a DIY recession, stressing how Britain was the fifth largest economy in the world and that it was strong enough to weather the economic turbulence that lay ahead.

But amid all the cries of woe and gnashing of teeth of the Remainers, the Brexiters are ecstatic.

Nigel Farage calls an impromptu press conference in a London pub. His voice cracking, he declares his mission accomplished. “We’ve got our country back,” he declares as he slurps on a pint of Spitfire ale. The Battle for Britain, he boasts, has been won.

Across the capital in Islington, the mood is quieter, more reflective.

Boris Johnson has yet to emerge from his plush five-storey townhouse. Interestingly, Michael Gove was spotted in the early hours discreetly entering the former London mayor’s home. A leadership bid to replace his Old Etonian chum appears already to be in the offing.

How did it come to this? The snap judgement was that the Vote Leave mantra of “take back control” stuck in people’s minds. Mr Cameron and the Remainers had underestimated the public’s deep concern about immigration and had no convincing riposte to it.

The anti-Establishment mood abroad that had helped Mr Corbyn become Labour leader and Donald Trump to become the Republican party’s presumptive presidential nominee had been channelled into a vote to leave the anonymous Brussels Establishment.

One of the key themes of the Scottish referendum campaign was change versus the status quo. Then, the No campaign realised in time that it was onto a loser if it was seen as defending the status quo and so quickly changed tack to insist a vote to remain in the UK was a vote for a changed relationship: more powers for Holyrood.

This time round Whitehall failed to move quickly enough and did not emphasise strongly enough the reform agenda while it had time. Too much effort and energy was spent on, as Mr Salmond colourfully put it, “scaring the bejesus out of people”.

Less than a week after the poll, Mr Cameron ironically finds himself in Brussels for a regular EU summit. He is commiserated by presidents Juncker and Tusk but some of his continental colleagues are not best pleased and want to give Britain a tough time so that, apart from anything else, contagion does not set in.

The PM had wanted to use the occasion to trigger Article 50 of the Lisbon Treaty, the law that begins the two-year process of divorcing Britain from the EU but the Brexit axis of Messrs Johnson and Gove persuade him to hang fire; they see no need to rush into an exit on Brussels' terms. In another illustration that power has already begun to shift significantly George Osborne signals that he intends to issue an emergency Brexit Budget, as promised before the poll, to hike taxes and cut spending in a bid to instil confidence in the markets that the UK had a contingency plan to steady the economic ship. But Messrs Johnson and Gove make clear the Brexiters won't buy it and, given the Tory majority is just 17, the chancellor has to backtrack; the rug is humiliatingly taken from beneath his feet.

Within days of the result nationalist stirrings in the Benelux countries and others hear calls for their own referendums. Anti-EU parties across the bloc are given heart, particularly in France, Germany and Holland, where elections are due in 2017. The question on everyone’s mind is: who will be next?

Back at Westminster and a month after the referendum result, the markets have finally calmed themselves as the Tories vote on their next leader and the country’s next prime minister.

Mr Johnson is the bookies’ favourite as Mr Osborne finds it hard to mount any kind of serious challenge, as he seeks to defend his record as Mr Cameron’s loyal lieutenant for Remain and to defend his doomed strategy of predicting economic Armageddon if voters were stupid enough to back Leave. He is “toast,” say colleagues.

Yet the Conservatives do what they often have in the past and choose the compromise “unity candidate”; someone whom Brexiters felt should have been on their side all along but reluctantly opted to stay loyal to Mr Cameron and someone whom Remainers felt could stop “Judas” Johnson from fulfilling his desire to take over the Tory party, having betrayed Mr Cameron.

Within minutes of the Conservatives' result, another figure is walking through the famous No 10 doorway to address the nation. Theresa May has become Britain’s only second female premier. A new, more uncertain era for Brexit Britain has dawned.