LONDON, ENGLAND - JANUARY 17:  Prince William, Duke of Cambridge and Catherine, Duchess Of Cambridge and Prince Harry seen leaving after a briefing to announce plans for Heads Together ahead of the 2017 Virgin Money London Marathon at ICA on January 17, 2017 in London, England.  Heads Together, Charity of the Year 2017, is led by The Duke & Duchess of Cambridge and Prince Harry in partnership with leading mental health charities.  (Photo by John Phillips/Getty Images)