Open this photo in gallery:Britain's Prime Minister Andy Burnham leaves 10 Downing Street in London on Sept. 9.Kin Cheung/The Associated PressAndy Burnham once said he would refuse to meet Donald Trump.Now Britain’s prime minister is about to hold his first meeting with the U.S. president, with Ukraine and the soaring cost of conflict in the Middle East at the top of his agenda.U.S. officials say the two leaders will hold talks on the sidelines of the United Nations General Assembly in New York. Burnham is set to address the annual gathering of world leaders on Tuesday, setting out his vision…