LONDON — By the time the Sunday newspapers went to press this past weekend, Prime Minister Boris Johnson of Britain had done something that he judged worse than lying “dead in a ditch”: asking the European Union to postpone Brexit, the project at the heart of his political being.

And yet Britons awoke on Sunday to front pages depicting a swaggering prime minister, tousle-haired and defiant, refusing to back down.

“Boris fights ‘Brexit wreckers’ with three defiant letters to EU,” The Sunday Times headline roared.

“Johnson refuses to sign Brexit delay letter,” The Sunday Telegraph blared, referring to a ploy that, along with the extra letters he sent, was promptly ignored by top European officials.