Four years ago, Nigel Farage decided he had achieved all he could in British politics. The gregarious English populist had campaigned for decades for Britain to leave the European Union, arguing the U.K. needed to make its own regulations and stop European migrants flowing in freely from the Continent to live and work. By 2021, Brexit was finally done. And so was Farage. “This has taken away the better part of my adult life,” he said as he announced he was quitting as leader of his anti-immigration party Reform UK. “I’m done.” Now Farage is back—and Reform UK is currently...