Paul Burrell might be best known as the controversial former butler to Princess Diana, but before he worked for the late royal, Burrell served Queen Elizabeth as a footman at Buckingham Palace. In a recent conversation, Burrell, speaking on behalf of Champions Speakers Agency, recalls a particularly memorable moment with the late Queen—and the joke that she didn’t quite get. Burrell, 68, shares that one of his duties was to bring the Imperial St…