SPOILER ALERT: This article contains spoilers for the season finale of “The Celebrity Traitors U.K.,” now streaming on BBC iPlayer. It is rare for the most popular show on television to be one of the best. It is rarer for that television show to become such a public spectacle that screenings of its finale fill out bars and clubs across the country. And it is even rarer for the tensions during those screenings, which you could cut with a knife, to be caused by the comedian Alan Carr . Yet “The