Top executives have warned U.K. Prime Minister David Cameron that he could damage Britain’s economy if he seeks to renegotiate the terms of its membership of the 27-country European Union. In a letter published in the Financial Times Wednesday, Virgin Group’s Richard Branson, London Stock Exchange head Chris Gibson-Smith, WPP chief executive Martin Sorrell and seven other business leaders challenged Cameron’s plan to renegotiate the U.K.’s membership terms of the 27-country EU and put the matter to a referendum.  The executives warn that such a plan could fail, pushing the U.K. out of the EU and hurting business in the process.  Article