Last year, Prime Minister Rishi Sunak said in a speech that he was proud to be Britain’s first prime minister of Asian heritage, but “even prouder that it’s just not a big deal.”On Friday, Mr. Sunak said he was “hurt” and “angry” after a man campaigning on behalf of Reform U.K., an anti-immigration party, was recorded on video using a racist slur to describe him. The same man also called for migrants to be used as target practice. The comments appeared in an exposé by Channel 4 News, in which an undercover investigator secretly filmed Reform campaigners in Clacton, a seaside area north east of London. The party’s leader, the veteran political disrupter Nigel Farage, hopes to win his first parliamentary seat there. The investigation, broadcast on Thursday night, raised uncomfortable questions about Reform, which has shaken up the country’s general election campaign ever since Mr. Farage reversed an earlier decision not to stand for Parliament.