LONDON — Keir Starmer looks headed for a bitter Labour leadership contest. And, like it or not, Britain’s relationship with the EU will be on the ballot paper. Britain’s prime minister is reeling from dire election results, and his potential rivals are circling. While no formal contest has yet been triggered, the shadow-boxing has begun. Ambitious former minister Wes Streeting has made his first move. The popular mayor of Greater Manchester, Andy Burnham, thinks he’s found a way into parliament.