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.  
U.S. will stand be steadfast ally to Britain as Brexit takes shape: Tillerson