Boris Johnson’s biggest challenge at COP26 doesn’t lie in avoiding a finger-wagging from Greta Thunberg, who won’t be going. Neither will it be in preventing the party being spoiled by Insulate Britain holding up the limousines of the great and good. Nor will Johnson have to struggle too hard to persuade his fellow world leaders to sign some kind of declaration strong enough to be spun as a triumph but anodyne enough to allow China, Russia and others to ignore it.