He did not put it quite like that of course. His reason, he said — barely keeping a straight face — was to introduce an urgent domestic agenda. But the effect was clear. Mr. Johnson has promised Britain will leave the European Union, deal or no deal, on Oct. 31. His opponents are combing through the system — its rules and conventions — trying to find ways to thwart him. Their latest idea was to force the government to seek an extension of the deadline. Mr. Johnson responded by simply shutting the system down.

This is, of course, extraordinary — a minority government deliberately preventing M.P.s from scrutinizing one of the country’s biggest decisions in memory — and steers close to authoritarianism. There is no conceivable democratic mandate for crashing out of Europe without a deal. Indeed, various members of the 2016 campaign to leave the European Union frequently ruled out this possibility. Mr. Johnson himself said a few months ago that the odds of no deal were “a million to one against.” Forecasters predict crashing out will do the country great harm.

Yet this is the strange place in which Britain finds itself. Here’s the problem. As with “Mornington Crescent,” the rules on which its politics are based are unwritten — in contrast to its French, American or German counterparts — and its smooth workings are held together by convention, good manners and a sense of whether or not something is fair play. There are also few correctives, it is finding out, should someone decide to break with all this.

As Mr. Johnson and the Brexiteers plow on, M.P.s and commenters have tried manfully to keep up with the game. “Ah yes,” they say, at every astonishing break with tradition. “Matters will soon return to normal. If you turn to Appendix VIII, Featherington edition …” (Even on Wednesday, there was debate on whether the Queen herself could or would intervene to end this breach of protocol. She could or would not.) The main players, though, have suddenly realized that they can win much faster by ignoring the rules altogether: “Mornington Crescent!” they chorus, immediately, and the game is over.

The rules are easy to ignore. Britain’s jumbled Constitution is set up that way, easily changed by a vote in Parliament or at a decision of the Speaker of the House of Commons. For years, British lawmakers were wary of tampering with it. But they have grown reckless.