LONDON (Reuters) - The United States will stand by Britain as it exits the European Union, U.S. Secretary of State Rex Tillerson said on Thursday after a meeting with British foreign minister Boris Johnson in London.  While Brexit does present unique challenges to the British people, please know that you have a steadfast ally in the United States,  Tillerson said at a news conference.  We will stand by our ally as Brexit continues to take shape. We look forward to continuing this long relationship.  