KYIV — Barely six months after his landslide victory in July, Keir Starmer has been hammered in the polls, beaten up by the financial markets and faced demands from the world’s richest man to be jailed. Swapping the political battles of Westminster for an actual warzone this week, the British prime minister found inspiration in the resolve of the Ukrainian people, and his stoical host, Volodymyr Zelenskyy. In an exclusive interview with POLITICO, Starmer scotched speculation about an early election, stating clearly that the next one won’t come before mid-2029. And he declared that he intends to win it and...