There is a lot of confusion about Boris Johnson’s approach to Brexit. And that is deliberate because the candidate has yet to make a big call about the nature of the modifications he is seeking to the Brexit plan negotiated by Theresa May.

The ultra Brexiters among his supporters, the hard core of the European Research Group led by Steve Baker and Jacob Rees-Mogg, want him to ditch her Withdrawal Agreement completely – and replace that with a “GATT 24” temporary free trade arrangement for the years that would be necessary for the negotiation of a permanent new trade deal with the EU.

This...