Jeremy Corbyn and Labour are playing Brexit games with their magical thinking on the EU

Remainers are right about one thing. No one voted for any particular model of Brexit. When Chuka Umunna, Nick Clegg, Anna Soubry and the rest say that there was nothing on the ballot paper about the single market or whether to stay in Euratom, they are spot on.

Binary simplicity is both the strength and the weakness of referendums. The purpose of a single-issue plebiscite is to set the parameters within which policy can be formulated. That's why, during the campaign, it never made sense to ask either side where they stood on fisheries, or budgetary reform or whatever.

A referendum is not a general election. People were not choosing Leave or Remain as their government. They were issuing a mandate to their politicians.

The Leave vote is best understood as an instruction – an order from the electorate to their representatives to withdraw from the European Union on the best possible terms. Plainly, people can sincerely disagree about what the best terms are. It would be odd if those who had campaigned to stay in didn't, by and large, want a closer deal than those who had campaigned to secede.

And in fairness, the narrowness of the vote gives Remainers an extra argument: if we are going to read anything at all into the result beyond its raw outcome, it is surely that the nation is evenly divided, and that we should aim at a relationship with the EU that is closer than that of a simple trade partner such as South Korea.

Alongside those sincerely arguing for soft Brexit, are those whose interest is not in the final settlement, but in making mischief. Some politicians think that if Brexit is made difficult enough, we will somehow drop the whole idea – a notion that they have scandalously encouraged in Brussels. Others seem not to have a fully thought-out strategy, but are so upset that they simply want to lash out.

Consider, for example, the motion in the European Parliament not to move on to trade talks. That wasn't about what kind of Brexit you wanted: it was a straightforwardly anti-British motion brought by Continental MEPs who were still smarting from the result. Yet, disgracefully, Labour and Lib Dem MEPs, along with two Tories, voted for it. By any definition, that was a wrecking device, not a difference in strategy.

Similarly, those British politicians who unhesitatingly backed the EU's absurd initial demands for an exit fee of £100 billion – a sum that even the most extreme hardliners in Brussels never seriously expected, but were putting on the table as an initial demand to be subtracted from – were not pursuing a different vision of where we should end up. They were simply jabbing angrily at the whole process.

Now here is the key question. In which category is Jeremy Corbyn's support for a customs union? Is it a legitimate proposal based on the idea of frictionless trade? Or is it a straightforward wrecking tactic?

For an answer, look at what Labour itself was saying until Corbyn's 26 February speech. Corbyn opposed the customs union on the impeccable grounds that it was "protectionist against developing countries".

Barry Gardiner, the shadow international trade secretary, saw it as "deeply unattractive", and explained one of the most serious downsides, namely the way it would apply to us asymmetrically: "The EU could do a deal with another country, let's say America, which we would be bound by in the UK, we would have to accept the liberalisation of our markets. Why would America give us that access when it's got all the liberalisation of our markets that it wants?"

Gardiner now seeks to answer his own objection by saying that the EU might offer us a completely different sort of customs union, one in which we negotiated everything jointly. And he has the gall to accuse the Tories of magical thinking on the EU.

What has changed? Only one thing. Labour thinks it has spotted an opportunity to win a vote on something. It hopes thereby to hasten an early election. Nothing surprising there, you might say. But what is surprising, is that Labour is playing games with the one part of the EU that the Left always found objectionable.

The EU's Common External Tariff, the basis of the customs union, creates two sets of losers: producers in developing countries and consumers in Europe, especially those on low incomes.

I understand that Labour, as a Left-of-Centre party, might feel warmer about some aspects of the Brussels system than I do. I get that socialists want EU functionaries to set our employment laws, eco-rules, safety standards and whatnot.

But there is no Leftist argument for a system that, as well as being "protectionist against developing countries", places the highest tariffs on the items that make up the largest share of the weekly bills of our poorest households, namely food, clothing and footwear.

Corbyn knows all this. He did his best to gussy up his U-turn with talk of being against contracting out and privatisation, but he is well aware that handing the EU control of our trade policy puts such issues beyond our control. Now, as EU members, we have a minority say in the commercial decisions taken by Brussels on our behalf. Under his scheme, Brussels would have 100% control and we'd have 0% input.

Kristian Niemetz of the Institute of Economic Affairs has a nice phrase. Leaving the single market while staying in the customs union, he says, would be like throwing away the burger and eating the napkin.

It's striking that not a single EFTA country wants to join the customs union. Yet Labour is now demanding a worse deal than theirs – indeed, the worst deal available. Its calculation seems to be that this is all terribly complicated, and we're a bit tired of Brexit, so it will get away with it. God help us if it's right.