(Reuters) – U.S. Secretary of State John Kerry entered 10 Downing Street with a bang on Tuesday, before expressing amazement at the rapidity with which new British Prime Minister Theresa May had taken up residence. Kerry, making his first visit to London since Britain voted last month to leave the European Union, hit his head on the door as he entered the official residence. Apparently unhurt, he later exchanged small talk with the Conservative leader, saying he was “amazed” at the speed of a transition that saw her move in to No.10 last Wednesday – right after David Cameron moved out. “It happened rather quickly … It’s such a different transition arrangement than in the United States,” May told Kerry. “I am amazed it happens so fast – how do you have time to pack everything?” Kerry replied at a photo opportunity where the two shook hands, smiled and sat down on matching armchairs in front of a period fireplace. Cameron quit after Britons voted narrowly in a June 23 referendum to leave the European Union, having failed in his bid to persuade them to back remaining in a bloc the United Kingdom joined in 1973. Emerging from the talks,