Andy Burnham became the UK's prime minister on Monday, capping an extraordinary demonstration of political power in which he easily won a seat in Parliament and swiftly engineered the resignation of the sitting prime minister so that he could take his place. Burnham, who spent the past nine years as the mayor of Greater Manchester, one of the fastest-growing regions in Britain, seized on widespread dissatisfaction with the former prime minister, Keir Starmer, to make his move.