To ask what Britain’s ‘national interest’ is has an old-fashioned ring to it. But in the new Brexit era, when we are supposed to be sailing the ocean blue, it is perhaps fitting; perhaps even something Brexiteers would cheer.

The ‘debate’ in the lead-up to the 23 June referendum hardly rose to the level of asking what was really best for the country. Instead the elevator got trapped between floors: was it this many millions a week, was it that. What does Steve Hilton say, who cares what Angela Merkel says. Do the Germans rely so much on car sales (and how badly now do the French want our jam).

Over the last fortnight politicians of different stripes have ventured their thoughts on what type of ‘deal’ (oh, how Trumpian that now sounds; look at the way our world is heading) Britain cuts with the European Union. Many from both Conservative and Labour parties have made clear that control over immigration from the EU is the red line we must draw on the ground. Others – fewer and more tentative, but finding their voice – say that the red line should instead be membership of the single market. Jeremy Corbyn, meanwhile, poleaxes everyone simultaneously (again) by backing free movement while spurning the single market.

Either red line may be presented as meeting the national interest – remaining in the single market would do so, most obviously, because of the threat to jobs and livelihoods that quitting it represents. MPs like Phil Wilson have come out in favour of this as the way forward.

On the other hand, the national interest may be ‘met’ by giving the people what they want – controls on immigration, no doubt one of the clearest reasons for the ‘No’ vote. If this is not provided, the British people’s faith in their own democracy will be so fundamentally shaken that the system will lose what already shrunken legitimacy it has.

Surely this is the axis on which the national conversation must spin? If you back controls on immigration, measure it up against the harm to the economy and then explain to the people why you still back controls. If you want to stay in the single market, explain how you can reconcile this with the desire of a sizeable proportion of the voting public to control – or, really, cut – immigration.

We are now witnessing the prime minister plunge down the latter course, while Brexit’s damage to the country remains blithely uninterrogated.

Only now are we having the debate that we, as a country, ought to have been having before June, long before in fact. This was a decision we drifted into over many years, in large part thanks to the failure of any political leader to square with the country about the give and take and sharing needed in the modern world. Politicians will soon have to pronounce on momentous questions in a way they have not been called on to do for decades, and their answers really will define the century to come.

They will also need to do this too in the knowledge that this conversation is going to knock the established political parties in all sorts of directions at once. Don’t expect party splits, but party splinters, along lines which bear little resemblance to previous cleavages. Inside Labour – remarkably – we need more, not fewer, siren voices which tell of the dangers ahead. If MPs genuinely decide that the loss of jobs and investment – perhaps not immediate, but a gradual seepage over years – is a price worth paying, then say so and square it with the electorate now. Voters are aware of the trade-offs. Despite the referendum, they have not genuinely been asked to decide between them yet. That time is coming now.

In 1848 Lord Palmerston told the House of Commons,

if I might be allowed to express in one sentence the principle which I think ought to guide an English Minister, I would adopt the expression of Canning, and say that with every British Minister the interests of England ought to be the shibboleth of his policy.

The question for every British MP is: what are the interests of the United Kingdom, and what is your policy?

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Adam Harrison is deputy editor of Progress and a councillor in the London borough of Camden. He tweets @AdamDKHarrison