At first, Nigel Farage kept his cool. When protesters disrupted an election victory speech by Mr. Farage, Britain’s veteran political disrupter, anti-immigrant activist and ally of former president Donald J. Trump, he ignored them.

But as the chaos persisted at the media conference on Friday, Mr. Farage began heckling back, drowning out critics by shouting “boring!” into the microphone no fewer than nine times.

With Mr. Farage around, things are rarely boring, however, as Britain’s center-right Conservative Party has just discovered to its cost.

Driven from power after 14 years by a Labour Party landslide, the Conservatives collapsed to their worst defeat in modern history, a stunning loss that has left the party’s remnants in disarray. By contrast, Mr. Farage’s small insurgent party, Reform U.K., is on a roll and has elevated him to a central determinant of the future of Britain’s political right — and perhaps the overall direction of the country.

His presence on the political scene, and his harsh, anti-immigration rhetoric, could have a crucial influence on the trajectory of the Conservatives, whose leader, the former prime minister, Rishi Sunak, said on Friday that he would stand aside once a successor was chosen.