Andy Burnham will be the next prime minister — confirmed, unopposed, and taking office on July 20 after 322 Labour MPs nominated him. But what he actually does with the keys to Number 10 is suddenly the most interesting question in British politics.

The PM-in-waiting has already signalled a sharp break. He's apologised for Labour's "too slow" response to Gaza, promising further sanctions on Israel and a potential ban on trade with illegal...