LONDON — A cluster of post-industrial towns in northwest England might be about to choose the country’s next prime minister. The people of Makerfield will on Thursday decide whether to vote Andy Burnham — mayor of Greater Manchester, often hailed “King of the North” — back into Parliament. If elected, he says he would challenge Prime Minister Keir Starmer as head of the ruling Labour Party — no public vote needed, thanks to the quirks of British politics. Two years after a landslide victory,