Today, I shall go to the German embassy in London to sign the necessary forms so that my half-German sons may apply for dual citizenship. My father and one of my brothers have already been granted Maltese passports (my family hails from the tiny Mediterranean island). I am glad that such options are available, as a Brexit of unknowable character approaches, and at a time when the prime minister sees fit to label those of us with funny continental names “queue-jumpers”. It is fair to say that, in the circumstances, we are fortunate.

Yet I think it is time to be a bit more honest and plain-speaking about those circumstances. For the most part, the debate about Brexit since the 2016 referendum has been framed primarily in economic terms. The leavers have spoken excitedly about the free-trade bonanza that supposedly lies the other side of 29 March. Remainers point out that Britain is cutting itself off from the largest single market in the world.

But this to-and-fro is a polite fiction draped over a much uglier reality, and one that has little to do with GDP. I keep hearing that it is rude and counterproductive to suggest that xenophobia played a significant part in the 2016 result, and it is certainly true that such accusations should never be made lightly.

All the same: there are moments when politeness is not always the highest priority, and reticence more closely resembles cowardice than civility. As Clint Eastwood’s Dirty Harry reminds us: a man’s got to know his limitations – and I’m afraid I’ve reached mine. I’m done with the whole pretence that the argument about Brexit is, and has always been, a subtle jurisprudential discussion about sovereignty, or an exciting debate about the business plan for UK plc.

Look at the evidence. In June 2017, a report collated from the British Social Attitudes survey showed that the most significant factor in the leave vote was anxiety about the number of people coming to the UK. A comprehensive study published by Nuffield College in April drew similar conclusions about the salience of immigration in attitudes to Brexit. “Take back control” was indeed the slogan of the leave campaign, but it was “control” with one purpose, above all others, at its heart.

Which is why Theresa May was so uncompromising last week when urged by cabinet colleagues that it was not too late to reconsider the impact of tighter immigration policy on British companies. An end to free movement, the prime minister informed them, was a non-negotiable feature of the deal that she will first put to the Commons on 4 December.

It is for this reason, too, that she is uninterested in the thoughtful proposal by the Conservative backbencher Nick Boles that she should contemplate a Norway-style interim arrangement with the EU, if MPs reject her agreement with Brussels. The Norwegian model preserves freedom of movement, which, for May, is the most garish of red lines.

I think something profound, unsettling but still underexplored, underpins this scale of priorities: a fundamental change that needs to be understood outside the day-to-day disasters of this useless government and even of the Brexit process itself.

Theresa May at the G20 leaders summit in Buenos Aires. Photograph: Marcos Brindicci/Reuters

For decades there was something close to a political consensus that the most important metric was economic prosperity. A wealthy nation was essential both to the aspirations of individual households and the funding of public services. The Tories might give greater weight to the former, Labour to the latter. Margaret Thatcher and Gordon Brown had radically different visions of social justice and collective responsibility, of the relationship between taxation and public spending. But they were as one in their conviction that nothing much was possible without a strong economy.

Brexit is both symptom and cause of a breakdown in this consensus. It can no longer be taken for granted that senior politicians, or the voters themselves, will automatically and reflexively put national wealth first.

As long ago as January 2014, Nigel Farage was explicit about this: “If you said to me, would I like to see over the next 10 years a further 5 million people come into Britain and if that happened we’d all be slightly richer, I’d say, I’d rather we weren’t slightly richer.”

Last week, the Bank of England warned that the UK was courting the worst recession since the 1930s. Philip Hammond was forced to concede that “the economy will be slightly smaller in the prime minister’s preferred version of the future partnership”, while the Treasury’s newly released forecasts suggest that the UK economy would be 9.3% smaller 15 years after a no-deal Brexit.

Of course, such statistics are now routinely dismissed as the latest iteration of “project fear”. But the more intriguing (and alarming) point is that so many senior figures in the political class responded with such comparative calm to these appalling prognoses. We have reached the point where, to an extraordinary extent, the implementation of the 2016 referendum result trumps all else.

Why so? Because, as Farage declared with more candour than most mainstream politicians can yet muster, culture is nudging old-fashioned political economy out of its prime spot. Immigration is now the gravitational centre of the whole debate: a debate much less about national wealth or national sovereignty than national identity.

Beneath all the talk of “control” and “global Britain”, there is the germ of an extremely unpleasant nativism. Again, we pesky centrists are told to be quiet and to heed the concerns of those who have been “left behind”. But since there is not a shred of respectable evidence that immigration has had more than a marginal impact upon public service capacity, wage levels or net welfare costs, I am forced to conclude that there is now a sufficiency of Britons who just don’t much like people of foreign extraction, and certainly don’t want many more of them around the place.

So often one hears that the British people “were not consulted” about immigration levels. To which the answer is: oh yes you damn well were. Every time you insisted on a properly staffed NHS, on social care that was halfway decent, on a service economy that worked, on affordable decorators, on your Tesco and Amazon deliveries arriving on time. Each time you took that landscape for granted, you were complicit in the immigration policy that preceded the Brexit vote.

We live in a world defined by the economic, social and cultural interdependence of nation states. And those who promise that leaving the EU will deliver “control” are really promising something quite specific: a social and cultural reboot. As well as being morally contemptible, of course, this is also a complete impossibility. But those who pose as our leaders have allowed this absurd and horrible vision of Britain’s future to take root. Let us be honest about what this is all about. And then let those who are responsible take full ownership of whatever consequences lie ahead.

• Matthew d’Ancona is a Guardian columnist