London (CNN) UK Prime Minister Theresa May is braced for what could be her biggest policy climbdown yet, after insisting for years that " no deal for Britain is better than a bad deal " and that her country would leave the European Union on March 29, 2019.

That road map is at stake this Tuesday, when lawmakers vote for a second time on her withdrawal agreement to take Britain out of the EU. If that agreement is defeated, they will be asked the following day whether or not the UK should leave without a deal -- an option May said should be kept for negotiating purposes.

And if that fails, MPs are likely to vote again Thursday on requesting a delay to Brexit from the EU.

May's beleaguered deal and its widely expected parliamentary defeat goes to show that the job of untangling more than four decades of European integration is struggling to live up to the lofty promises of the 2016 referendum campaign.

The hubris and fantastical claims sold by many Brexit-supporting politicians -- and indeed some remainers -- are now facing their moment of truth as the deadline for the UK's withdrawal from the EU looms large.

The sunlit uplands

They initially said it would be so easy. The Vote Leave campaign and its supporters claimed that negotiating with the EU would result in the " easiest trade deal in human history ." The vote would put the UK in " a free trade area stretching from Iceland to Turkey ," said prominent Brexiteer Michael Gove -- now a senior member of the government.

But even before the June 2016 referendum vote, the rhetoric was being picked apart. Johnson claimed -- in numerous editorials, speeches and interviews -- the weekly sum of £350 million ($456 million) paid to Europe could instead be used to finance Britain's beloved, but overstretched, National Health Service.

Hello, I am from Britain, you know, the one that got tricked by a bus — Ahir Shah (@AhirShah) February 8, 2017

That figure was lampooned as " potentially misleading " by the nation's statistical body, as it did not take into account the rebate applied before Britain pays its contributions to the EU -- or other benefits it receives.

The gloomy lowlands

Fast forward to 2019, and the many promises are under even more scrutiny.

Estimates now suggest that the weekly cost of leaving the EU is more than double the £350 million famously emblazoned on the side of the Leave campaign's battle bus.

Bank of England economist Gertjan Vlieghe said in a February speech that Brexit had cost £40 billion ($52 billion) a year since the 2016 vote -- a weekly loss of £800 million ($1 billion).

Other aspects of the Brexit campaign now ring hollow, with trade minister Liam Fox possibly regretting his prediction that a trade deal with the EU would be " one of the easiest in human history ." Even supporters admit there will be at least short-term disruption, and claims of "sunlit uplands" have been replaced by assurances that Brexit "will not be the end of the world," as May said last August on a trade mission to Africa

Brexiteers campaigned on the idea they could "take back control" of Britain's borders and immigration controls, and "make our own laws," while retaining all the economic benefits of the single market. "Our policy is having our cake and eating it," as Johnson declared . Exactly how this was possible remained unclear.

"Cakeism," as the Brexiteer approach came to be known, failed to recognize that the European single market came with its "four freedoms" -- the free movement of goods, services, capital and, most importantly, people.

And leaving the customs union came with major complications, namely the creating of a hard border between Northern Ireland and the Irish Republic that would contravene the 1998 Good Friday agreement that aimed to end decades of sectarian violence.

And what Brexiteers underestimated was the unity of European leaders during negotiations -- and how Northern Ireland, and the Conservative party's governing partner, the Democratic Unionist Party of Northern Ireland (DUP), would be a huge source of constraint.

By September 2017, Brexit Secretary Davis admitted negotiations with the EU were tougher than he'd thought. "Nobody has ever pretended this will be easy. I have always said this negotiation will be tough, complex and at times confrontational," he said.

The Brexiteer response

In 2018, it became clear that May's "red lines" of leaving both the single market and customs unions were softening toward a divorce deal that would indefinitely bind the UK closer to the EU's rules. Davis resigned as Brexit secretary that July to protest May's new customs plan.

The man who negotiated Brexit resigned today to position himself to lead the party he wants to negotiate against the deal he negotiated. — Armando Iannucci (@Aiannucci) November 15, 2018

He was followed out of the Cabinet the next day by Johnson . Johnson's resignation highlighted Brexiteer politicians' new and relaxed view of leaving the EU with no deal and trading under World Trade Organization (WTO) rules -- a scenario barely mentioned during the referendum campaign.

Leave supporters " voted to come out ," Johnson wrote in his weekly column for The Telegraph this January, advocating for a hard exit from the EU.

Theresa May accepting resignations over Brexit https://t.co/zFvChz5NvV — kelly oakes (@kahoakes) November 15, 2018

"It is no deal, or WTO terms, that actually corresponds to their idea of coming out, and they view that option with a confidence that is now directly proportional to the growing strength of the government's warnings against it."

From global Britain to lonely Britain

Among the many climbdowns was Britain's ability to strike free trade deals with any country of its choosing -- something it cannot do while still in the customs union.

Davis had claimed, for example, that it would take only two years for Britain to "negotiate a free trade area massively larger than the EU."

Not only would these "new trade agreements ... come into force at the point of exit from the EU, but they will be fully negotiated," he said in a 2016 Conservative Home piece

That year, Brexiteer Liam Fox was tasked to do just that with the newly created title of international trade secretary. He promised that "one second after midnight in March 2019" the UK would have 40 trade deals in the bag.

But with the clock ticking toward that very midnight, local reports said he had signed post-Brexit trade agreements with six governments , among them Switzerland, the Faroe Islands, the Palestinian Authority and Chile.

Culture war

Immigration was the hot-button issue that drove the Brexit vote; the Office of National Statistics (ONS) ranked it British people's main preoccupation in the spring of 2016.

That view was encouraged by Vote Leave claiming that remaining in the EU would mean up to 5.23 million more immigrants coming to Britain.

In the weeks before the referendum vote, Brexit-supporting politicians, including Penny Mordaunt and Michael Gove, had falsely stated that Britain would be exposed to a wave of Muslim immigration by Turkey joining the EU.

"With the terrorism threat that we face only growing, it is hard to see how it could possibly be in our security interests to open visa-free travel to 77 million Turkish citizens and to create a border-free zone from Iraq, Iran and Syria to the English Channel," Gove told a Vote Leave event.

The Committee on the Elimination of Racial Discrimination, a UN body, accused British politicians of fueling a sharp rise in racist hate crimes during and after the EU referendum campaign -- something right-wing newspapers and commentators continue to claim is a myth.

Welcome to the future. pic.twitter.com/8U3DgI4kgF — David Baddiel (@Baddiel) March 29, 2017

Gove now expresses regret and said it was wrong of the campaign to fuel immigration fears about Turkey. "If it had been left entirely to me, the Leave campaign would have had a slightly different feel," he said, according to Tom Baldwin's 2018 book, "Ctrl Alt Delete: How Politics and the Media Crashed Our Democracy."

In 2017, May promised to slash the number of migrants to the "tens of thousands" and to end free movement -- the right of Europeans who live in the EU to move to other member states. However, the attempt to do so was abandoned during negotiations.

But by that point no one was listening. According to the spring 2018 ONS statistics , immigration concerns had lost their bite -- ranking below housing, cost of living, and health and social security.

A tale of two sides

The Remain camp was not free from miscalculations, either. When the Leave campaign won, David Cameron resigned as prime minister -- after saying he would stay on.

Leave voters also say Remain predicted a "Brexaggedon" that has not come to pass. There is an element of truth in their claims. Former Chancellor George Osborne warned that a leave vote would lead to "an immediate and profound shock to our economy. That shock would push our economy into a recession and lead to an increase in unemployment of around 500,000."

The economy did not contract and the level of unemployment is now the lowest on record. But it was only the Treasury that wrongly predicted a recession.

By 2018, UK's economy expanded at its slowest annual rate in half a decade and the British pound is now 14% lower than before the referendum. It comes amid strident warnings that businesses would struggle or relocate after Brexit.

Not quite an apocalypse, but nor does it signal Britain is making what Boris Johnson once declared would be a " titanic success " with Brexit.

Amid the predictions and confusion, one thing is certain -- if Brexit does go ahead, it will look nothing like the vision that the British people were originally sold.