CLACTON-ON-SEA, England—British parliamentary elections have not been kind to Nigel Farage. He’s previously failed in no fewer than seven attempts to win a seat in Westminster, one of which nearly killed him, but he remains undeterred. On Thursday, as U.K. voters head to the polls in the general election, the right-wing populist Reform U.K. leader—promising MAGA-style immigration crackdowns and a “patriotic” curriculum in schools—might...