In 2016, more than 17 million British people voted to leave the European Union. But – as the journalist Tim Shipman’s new book, Fall Out, which covers the political turmoil of the 15 months since the referendum, makes clear – only two British people decided that this meant leaving the European single market, the customs union and the jurisdiction of the European court of justice.

Those two people were Theresa May and her former aide Nick Timothy. These fateful national decisions were their personal interpretations of the vote to leave the EU – and theirs alone. As Mr Shipman explains, these foundational decisions of the UK’s withdrawal strategy were not discussed by Mrs May’s cabinet, let alone by parliament. Instead they were simply prime ministerial edicts to the 2016 Conservative conference. Later, they were included in the Tory manifesto for the June 2017 election, in which the party lost its overall majority, Mrs May’s leadership was humiliated and Mr Timothy lost his job.

They were and are reckless and foolish policies, not justified by the 2016 referendum outcome, at odds with the national economic interests of Britain, inimical to peace in Ireland, and threatening to the internal cohesion of the UK. They are largely responsible for the mess into which Mrs May led the British negotiating effort in Brussels this week. The choice to leave the market, which is a project that had been shaped by Britain for more than 30 years, and the customs union collided on Monday with the reality that to do so will inflict a hard border inside Ireland or create a new border in the Irish Sea between Ireland and Britain.

On Tuesday, while ministers and officials in London, Brussels and Dublin tried to piece back together a version of the deal that had been vetoed by the Democratic Unionists, MPs from all parties at Westminster made clear that neither of these options is acceptable. The MPs are right. Cobbling together a form of words that will pass muster with the EU, the Irish government, Tory backbenches and the DUP may get Mrs May over the immediate negotiating hurdle in Brussels next week. But in the long term, even if Brexit goes ahead, it is not sufficient. The policy itself needs to change.

In a Commons performance marked by too much complacency and condescension, the Brexit secretary, David Davis, continued to take the cobbling option. His aim is “regulatory alignment” between the EU and the UK (the Irish Sea option has been ditched) rather than convergence or harmonisation. He seeks “mutually recognised rules, mutually recognised inspection”. But this is to offer a superficial and semantic solution to a deep and material problem. Without harmony there is only disharmony or stasis. The economy, jobs and the Irish issues all require a clearer and more engaged approach.

That approach is to stay in the customs union and to keep the single market on the negotiating table, as Labour significantly now advocates. Labour’s tougher stance is principled. It marks a clear rejection of the deregulatory liberalism that lurks behind so much Brexit thinking. Tuesday’s Commons exchanges strongly suggest there is in fact a majority in parliament – as there is in the country – for doing this. Staying in the customs union as the best way of protecting the UK economy and avoiding a hard border in Ireland was backed not just by most Labour MPs, but by several Tories and most other parties. Even the DUP could probably live with it. “We are as one,” said the Tory Anna Soubry. “Just do it,” said Labour’s Chris Leslie. Do it indeed. It is the policy whose time has come.