LONDON — Ukrainian President Volodymyr Zelensky was in Britain Monday on his whirlwind European tour, as the staunch ally of Kyiv promised to give Ukraine hundreds more missiles and attack drones in an effort to change the course of the war. Zelensky landed by helicopter at Chequers, the British leader’s official country retreat, and was greeted by Prime Minister Rishi Sunak with a handshake and a hug.