Queen Elizabeth II and US President Barack Obama review an honour guard of the First Battalion Scots Guards at a ceremony of welcome at Buckingham Palace on May 24, 2011 in London, England. The 44th President of the United States, Barack Obama, and his wife Michelle are in the UK for a two day State Visit at the invitation of HM Queen Elizabeth II. During the trip they will attend a state banquet at Buckingham Palace and the President will address both houses of parliament at Westminster Hall. (Photo by Murray Sanders - WPA Pool/Getty Images)

Queen Elizabeth II and US President Barack Obama review an honour guard of the First Battalion Scots Guards at a ceremony of welcome at Buckingham Palace on May 24, 2011 in London, England. (Photo by Murray Sanders - WPA Pool/Getty Images)

U.S. President Barack Obama (L) and his wife Michelle (R) are greeted by British Prime Minister David Cameron and wife Samantha Cameron (2nd L) outside number 10 Downing Street on May 24, 2011 in London, England. (Photo by Jeff J Mitchell/Getty Images)

US President, Barack Obama, first lady, Michelle Obama, and the Dean of Westminster Abbey, John Hall pay their respects at the tomb of the Unknown Soldier in Westminster Abbey on May 24, 2011 in London, England. (Photo by John Stillwell - WPA Pool/Getty Images)

U.S. President Barack Obama and British Prime Minister David Cameron (R) play table tennis at Globe Academy on May 24, 2011 in London, England. (Photo by Richard Pohle - WPA Pool/Getty Images)

(L-R) Queen Elizabeth II poses with U.S. President Barack Obama, his wife Michelle Obama and Prince Philip, Duke of Edinburgh in the Music Room of Buckingham Palace ahead of a State Banquet on May 24, 2011 in London, England. (Photo by Chris Jackson - WPA Pool/Getty Images)

U.S. President Barack Obama and Queen Elizabeth II during a State Banquet in Buckingham Palace on May 24, 2011 in London, England. (Photo by Lewis Whyld - WPA Pool/Getty Images)

(L-R) U.S. President Barack Obama, Queen Elizabeth II and First Lady Michelle Obama arrive at Winfield House, the residence of the Ambassador of the United States of America, in Regent's Park, on May 25, 2011 in London, England. (Photo by Yui Mok - WPA Pool/Getty Images)