Britain's Queen Elizabeth II, centre, walks down the staircase in 10 Downing Street past photographs of previous prime ministers with current Prime Minister, David Cameron, and his wife Samantha, right, after they had lunch together Tuesday June 21, 2011 to mark the 90th birthday of the Duke of Edinburgh, unseen, the queen's husband. Previous prime ministers are: top row from left, Edward Heath, 1970-1974, Margaret Thatcher, 1979-1990, and Tony Blair, 1997-2007, bottom row from left, Harold Wilson, 1964-1970 and 1974-1976, James Callaghan, 1976-1979, John Major, 1990-1997, and mostly obscured, Gordon Brown, 2007-2010. (AP Photo/Stefan Rousseau, Pool)