Before he even entered the race, Andy Burnham was the favourite to unseat Keir Starmer as Britain’s prime minister. The mayor of Greater Manchester led opinion polls and was the front runner among bookmakers and Polymarket. Late Thursday, the biggest obstacle fell away: Labour MP Josh Simons quit the House of Commons, clearing the way for Burnham to win a seat in Parliament, a requirement for the top job. Arriving in London this week as Starmer’s government wobbled, Burnham has kept a low...