LONDON — The Iran war has left Britain’s armed forces exposed, heaping pressure on Prime Minister Keir Starmer to act on his promises to invest in defense, after years of warnings from military bosses about the U.K.’s shrinking capabilities. When a British military base in Cyprus was hit by a drone early on in the Iran conflict in March, Britain, whose navy was the largest in the world at the start of World War Two, took three weeks to deploy one warship to the eastern Mediterranean. France,