Boris Johnson’s political death should have happened a decade ago when, as London Mayor, he agreed to take part in a stunt from which any other politician would have fled.

On a visit to promote a newly opened adventure park, he allowed himself to be harnessed to a zipline on which he inelegantly descended with a Union Jack in each hand. The line had been poorly tensioned causing Johnson’s portly form to come to an undignified stop someway short of the gantry from which he was to make his escape. He hung there for...