From the moment Britain voted for Brexit in 2016, there was only one way to go. It was back to the European Free Trade Association (Efta), of which the UK was a member before 1973. Nothing else made sense. As a Eurosceptic, I voted to remain only because I thought it wrong for Britain to leave Europe’s one conclave of nations just when it was growing seriously unstable. The vote to leave had to be honoured. It was doable. But to abandon the customs union and single market was not doable. It was reckless.

I assumed – and was told – that from day one, Theresa May’s negotiating officials were of the same mind. So too were the majority of MPs and the drift of public opinion, whenever polled. All were against hard Brexit and in favour of varying versions of the single market, in other words the “Norway” model. But May scuppered any hope of cohering such a coalition, by appointing hardline ministers to the Brexit brief and then giving in to them with her red lines in the ill-judged Lancaster House speech in 2017. Instead of talking the language of compromise, which she must have known she would later have to do, she narrowed her room for manoeuvre.

Any deal to take the UK out of the EU’s customs union was going to founder on the Northern Ireland question. It is geographically part of Ireland, an EU country. If the UK wants its own separate customs arrangements, a border must be declared somewhere. You cannot square a circle. That is why the EU’s legal position on the deal is understandable. If Britain wants an open border within Ireland, it must say where its newfound sovereignty begins, presumably in the Irish Sea. In other words, staying in the customs union was always the only way through the Northern Ireland thicket.

The Democratic Unionist MPs know this perfectly well – which is why, if they vote against May next week, no one should ever trust their word again. I would tear up the Treasury subvention.

Despite May not even trying to mobilise what would be a Commons majority in support of the customs union, MPs must see that she has, in effect, kept the customs union option open under her transitional deal. That the hard Brexiters so fear it should give the soft Brexiters – and remainers – genuine comfort. The real negotiations on a Norway deal only begin in April.

The last time Norway hovered over the House of Commons was after its invasion by Hitler in 1940. This defeat for the allies split the Tory party and felled its leader, Neville Chamberlain. His demise was the price demanded by Labour for a national coalition. If May loses theCommons vote next week, a rerun of 1940 is eerily possible.

She would have to engineer a second vote. She could desperately go to Brussels and seek some cosmetic easing of the “backstop get-out”. The noises from the EU on this are pessimistic, but it would be a politically sensible move. And/or she might discuss with opposition parties a Commons grand committee, to reach an agreed motion for a second vote, perhaps under the chancellor, Philip Hammond, and Labour’s Sir Keir Starmer.

If May won a second vote, she would be safely through to March. If she lost, then thick fog would roll in from the Thames. Twice would her Brexit strategy have been voted down, and from the pro-EU side as much as from her own right wing. She would either resign or offer herself up for party re-election, as John Major did in the summer of 1995. The latter she might win, her MPs knowing that toppling her would mean a vote of no confidence and probable general election. A furious electorate might not like that, not to mention the monarch, faced with finding a prime minister from two bitterly divided parties in the Commons.

Even from the resulting mess, I still see only one sensible conclusion emerging, one that reflects the bald fact that neither the public nor most MPs want the UK to leave Europe’s single market. Parliament’s task will be to find a way of saying so. For an offshore island to cut itself adrift from “frictionless trade” with its mainland is gross self-harm. Unscrambling half a century of collaboration with an ever more integrated Europe is pointless. The lobbyists are not all fearmongers – they are from business, industry, the City, the police, the NHS, the care sector, science, universities and tourism. They plead unanimously with MPs to keep the UK in the community of Europe, represented with Thatcher’s single market.

Some objections to Efta/Norway are footling, that it would “have to be permanent” and it involves “vassalage”. All trade deals are temporary and their terms are a function of power, as Donald Trump is showing. The idea that Britain would be “rule-making” in trade with the US is absurd. Ask China. Changes in migrant rights under the single market would need renegotiating, but such changes are happening across Europe. The freedoms of the single market make sense. No hard Brexiter has been able to quantify any benefit from “making deals with the rest of the world”. It is pseudo-chauvinist rhetoric.

Norway is not ideal, but it is workable. It is now the topic of discussion wherever reasonable MPs gather together. Europe’s economic area is where Britain will one day forge new links with a reorganised Europe, as so often in the past. The question is how much blood is spilled in the process.

• Simon Jenkins is a Guardian columnist