LONDON — The U.K. is too dependent on the United States for its defense and has not adequately invested in a back-up plan should the U.S. pull away from Europe, a damning report by a cross-party group of lawmakers warned Wednesday. A report by the House of Commons Defence Committee says the U.K. could be left in a vulnerable position if a global crisis sees America pull away from its traditional role of militarily backstopping Europe. Britain needs to see where it could replace U.S.