Among the multiple absurdities uttered by those who demanded Britain’s departure from the European Union is the claim that, since the sky has not yet fallen in, all those gloomy warnings from the remain crowd have been proved wrong. Absurd because – and it’s odd that they haven’t spotted this – we have not yet left. We remainers believed that it was the actual leaving, not a mere vote to leave, that would bring economic havoc. That the first few post-referendum months seemed steady enough can be attributed to the hope nurtured by some of our trading partners, along with the markets, that we might not go ahead with this planned act of national self-harm, that we might step back from the brink.

Which is why our currency takes a plunge every time Theresa May or her ministers indicate that, no, Brexit really will happen. The harder Brexit threatens to be, the deeper the pound tumbles. This week sterling has been a veritable Tom Daley, somersaulting downward. Next time a leaver cheerily tells you that Brexit has had no economic impact, you don’t have to just roll your eyes and remind them we’re still in the EU. You can tell them that we’ve endured an involuntary currency devaluation to the tune of at least 13%. That helps some exporters, but it’s also made Britons poorer.

Of course, the referendum result has to be honoured. But the only instruction the British people gave on 23 June was to leave the European Union. They did not issue an edict demanding the most extreme rupture possible, one bound to imperil our economy and rip away at the fabric of our society.

Yet that’s what our politicians seem bent on. Right now, the government could have been devoting itself to a diplomatic offensive aimed at persuading the EU’s 27 other members to grant us a Norway-plus arrangement, allowing us to enjoy the benefits of single market membership with a degree of flexibility on how free movement is implemented. (Bear in mind that not all the four core freedoms are applied absolutely: there are exceptions to the free movement of services, for example. Recall too that what Europe originally enshrined in treaty was free movement of labour, not people: that too could allow for some British leeway.) Yes, it would have been a big ask. But with the moral mandate of the referendum behind her, Theresa May would have been hard for the EU 27 to refuse outright.

Instead, the government keeps slamming its fist on the table and insisting on the worst possible deal for Britain. No to the single market, says May. Liam Fox wants to go further, exiting the European customs union as well. Economists warn that if we do that, it won’t just be jobs, investment and people that will steer clear of Britain the day after we leave: basic food imports won’t be allowed in either. We currently lack the basic trade agreements we’d need on day one outside the EU. They will not fall into our lap automatically, and will take a lot longer than two years to negotiate.

Meanwhile, we are methodically trashing our brand, the very international reputation that has made us an attractive place to invest, visit or live. Most countries long to bring in the best and the brightest. We seem determined to turn them away. This week the government signalled to foreign-born doctors and students that they are no longer wanted here, as well as warning diverse, global companies they will be named and shamed for the crime of drawing on a wide, international pool of talent. On Friday the London School of Economics claimed the government had told its non-British experts on Europe that their wisdom was not wanted. The Foreign Office rejected that claim. But a desire to purge ourselves of foreigners does seem to be turning into a fever: hot, irrational and ugly.

And all this while telling EU residents who have made their homes in Britain that they are to be “one of our main cards” in future negotiations with the EU – as if, should Britain not get what it wants, it might actually deport up to 3 million people who came here legally and in good faith. It’s one thing to listen respectfully to those millions of leave voters who want a say over how future migration works. It is quite another to demonise, and terrify, those who are already here.

To do all this, May has had to rewrite the history of 23 June. No longer the winner in a fairly narrow vote of 52% to 48%, leave was recast in the PM’s conference speech as a mass consensus, the unambiguous stance of the great British public. All that had ever stood in their way was a liberal, monied elite, the Davos set of footloose globetrotters devoid of national allegiance, citizens of the world and therefore citizens of nowhere. Before our very eyes the 48% are being rebranded as the 1%.

What’s so odd about this is that it is former remainers who are doing it. Recall that May stood – not very visibly, it’s true – alongside those who wanted to stay in the EU. Amber Rudd, she of the foreigners’ lists, confronted Boris Johnson in a TV debate. Jeremy Hunt, the health secretary now eager to denude the NHS of the world’s best medical talent, was a remainer too. They are engaged in a collective act of over-compensation, frantically mouthing the prayers of the new religion now that the old one has been banished as heresy.

So who will speak for the 48%? They – we – are not looking for a champion who will pretend that the referendum never happened, but one who will fight for a Brexit that does not deprive us of all we cherished in our relationship with Europe. It means arguing for a sane, practical deal that serves, rather than harms, our national interest – and that could then be put to the British people in an election or even, if that’s what it takes, another referendum. It means challenging, for instance, Fox’s ludicrous, dogmatic search for commerce with every far-flung corner of the planet except the 500 million-strong market on our doorstep with whom we currently do nearly half our trade.

The key arena for this battle will be parliament. Labour has a national duty to challenge the government’s every step: the combination of Keir Starmer, shadowing the Brexit ministry, and Hilary Benn chairing the relevant Commons committee could deliver sharp scrutiny. But they are impaired by the Labour leadership’s perverse readiness to jettison single market membership, even as it admirably defends free movement.

Labour will need allies. The Lib Dems are too few, though on this Nick Clegg is a serious asset. Tory remainers still outnumber leavers in the Commons, but they’ve gone quiet, with a few laudable exceptions. They need to rediscover their voice – not least because, as our politics currently stands, it is the challenge from within that threatens May most.

Those MPs, of all parties, have a solemn responsibility. But it cannot fall on them alone. We the 48% have to speak for ourselves – and halt this march into madness.

• This article was amended on 11 October 2016. An earlier version said that “what Europe has enshrined in treaty is free movement of labour, not people”. The original treaties applied only to free movement of labour, but free movement of people was established by the Maastricht treaty.