LONDON — The last dregs of Keir Starmer’s authority were draining away even before his likely replacement could board the Monday morning train to Westminster. Britain’s prime minister is widely expected to announce a timetable for his exit on Monday, less than two years after a landslide election victory returned his center-left Labour Party to power after 14 years. His putative successor — Andy Burnham, the outgoing mayor of Greater Manchester — would become Britain’s seventh leader in a