WASHINGTON (Reuters) - U.S. first lady Michelle Obama will join her husband, President Barack Obama, for his trip to Britain later this week, the White House said on Monday. The Obamas will attend a private lunch with Queen Elizabeth and her husband, Prince Philip at Windsor Castle on Friday.  That evening they will have diner with Prince William, his wife Kate, and Prince Harry at Kensington Palace, White House spokesman Josh Earnest said. The president’s trip comes as the British weigh whether or not to remain in the European Union in an upcoming referendum and as Obama completes his final year in office as president.  