Rarely has Britain’s new political landscape come into sharper relief than during the rapid-fire events that unfolded over a hectic lunchtime on Friday.

At noon, 10 Downing Street announced that Angela Rayner, the deputy prime minister in the Labour government, had resigned after admitting she had paid too little tax on the purchase of a seaside apartment. An hour later, Nigel Farage, the leader of the right-wing anti-immigrant party Reform U.K., hurried to the stage at the party’s annual conference in Birmingham, England, to exult in Labour’s misfortune.

Ms. Rayner’s resignation was not a major shock after two weeks of questions about her tax problems. But its timing, on the opening day of the Reform conference, gave Mr. Farage a ready-made moment to press his populist case: that the Labour Party and its prime minister, Keir Starmer, are aimless, incompetent and unfit to govern.

“We are the party on the rise,” Mr. Farage, an ally of President Trump, said to a cheering crowd in a cavernous convention center, with pulsating lights and flashy pyrotechnics. “I frankly, myself, couldn’t believe just how well we’ve done.”

Mr. Farage’s triumphalism glossed over some less convenient truths. He leads a party that has only four out of 650 members in the House of Commons, a meager fund-raising record, virtually no experience governing and policy positions that alarm large parts of the voting public.

Still, the diverging fortunes of Reform and Labour are impossible to dispute and increasingly hard to ignore.