Boris Johnson was right: group debates are awful. Last night was Love Island with one-liners, five rivals pouting and shouting at the same time. Since viewers have no voice or vote in the outcome, what was the point? Job interviews should be conducted individually, not as a group.

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Of the five candidates who showed up, three simply lied. They claimed to have their own unrevealed formula for “renegotiating” a Brexit deal by October. A fourth, Dominic Raab, had been Britain’s lead negotiator. He knew that this was rubbish and would rather support no deal, chaos and all. The fifth, Rory Stewart, was the boy who said the no-deal emperors had no clothes. The only way to leave the EU was Theresa May’s deal, for which the others had actually voted. They were hypocrites who should all now work to get it accepted. Stewart, a rank outsider, was not so much the winner as the only honest man standing.

For good measure, Johnson had given a far more revealing interview to the BBC last Friday, exposing the “renegotiate by October” strategy as piffle and wind. He, too, claimed to have a cunning plan for a borderless border, in which flows of goods, services and people would be allowed in but policed “at the point of sale”. He sounded barmy. With the nation’s farming, food, engineering and pharmaceuticals industries just four months from massive disruption, and with billions of pounds being wasted on no-deal preparation, it is appalling that Britain’s ruling party can do no better than this.

The issue of the Brexit deal must be confronted this coming week. Nothing else matters. Stewart’s challenge must be answered: how on earth can the candidates claim they can improve on May’s deal by October, when they know they cannot? There must be a backing down and a House of Commons agreement on a deal to take back to Brussels urgently. The leave campaign clearly promised voters that it would deliver “frictionless trade” in the event of EU departure. There is no other definition of that but “soft Brexit”. Stewart is the only candidate ready to honour that promise. If these debates mean anything, he wins.

But they mean nothing. The result is that Britain’s leadership now turns on which candidate can be relied on to back down fastest from this week’s populist bombast. Whose record of double-talk, about-turn and mendacity will prove the most robust?

The answer appears to be a certain elusive Boris Johnson, whose handlers are too terrified of his unreliability to allow him public exposure. He is the one whose flexibility with the truth is most inflexible. His untrustworthiness is the one in which the nation can put most trust. The nation is about to be governed by a contradiction in terms, an oxymoron-in-chief.

• Simon Jenkins is a Guardian columnist