Andy Burnham became Britain’s seventh prime minister in a decade on Monday, pledging to break with the politics of the past to deliver a new economic model to benefit a nation “fed up” with a revolving door of leaders. In front of dozens of supporters outside his new office and residence at 10 Downing Street, Burnham said politicians had failed to bring the stability needed to raise living standards, something he said he would change with immediate policies and a longer-term plan to be drawn up...