Here we go again. Brexit did not end on Friday night. Formal divorce proceedings reached a messy conclusion, but the couple will cohabit for at least another 11 months. Nothing in practice has changed. No one is hurt, yet. Anything might still happen.

A helpful sign has been the hopes expressed by sensible Europeans such as the former EU president, Donald Tusk, for friendly relations to be restored. Positive trade arrangements are mutually advantageous. There is no gain in damaging them for the sake of political grandstanding. Surely now we can be grown up.

Trade is about muscle. The UK is about to lose the muscle of EU collective negotiation

It is the more baffling, then, that Boris Johnson should choose the week after his divorce celebrations to hurl belligerent remarks at the EU, orchestrated with his foreign secretary, Dominic Raab, and the chancellor, Sajid Javid. The hubris of election victory has gone to their heads. Like many a divorcee, Johnson cannot resist trying to humiliate his former partner.

There is simply no point in the UK risking 50% of its trade, for that is how much is done with the EU. Yet Johnson declares he would actually prefer a tariff wall with the EU against any continuance of the present single market. He wants “no regulatory alignment” with the rest of Europe, “no level playing field”. He demands a relationship similar to that the bloc has with Canada, which is minimally integrated with the EU, or even that of Australia. The latter is code for no deal at all and the imposition of an EU tariff wall. If that is where the country ends up, says Johnson, “I have no doubt that Britain will prosper.”

There is no evidence for this madness. There is no way some notional boost in trade with China or the US can compensate for what “hard” Brexit would cost the economy. Nor will such trade reflect any vaunted increase in sovereignty. Trade is about muscle. The UK is about to lose the muscle of EU collective negotiation. Any marginal rise in trade with China or the US will be strictly on their terms. As Johnson would say, it will be vassalage.

Of course, much of Johnson’s language is political chest-beating. Continuing to deal with the EU at all requires agreements on trade and movement of people. If it helps Johnson’s paranoid ego to reject “legislative” alignment or “rule taking” or “level playing fields”, perhaps he can be textually humoured. Negotiators should be able to reach sectoral agreements on goods and services, covered by enforceable protocols ensuring fair competition. The City has already reached such temporary deals on money markets. Likewise many EU countries exercise sovereignty over migrants’ employment and welfare rights.

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What is not the case is that somehow the UK has overnight become a more potent negotiator than before. Trade is not about sovereignty. It is about commercial and economic weight. In this respect, Britain outside the EU has been weakened, not strengthened. The lion has become a mouse, a mouse trying to roar. It may make Johnson and his colleagues feel good. But they owe it to the country to swiftly reach a free trade deal with the EU, and that means free.

• Simon Jenkins is a Guardian columnist