LONDON (Reuters) - A group of British business chiefs called on Thursday for a referendum on whether to leave the European Union on the terms that Prime Minister Theresa May hopes to negotiate or to stay in the world's biggest trading bloc.Less than five months before Britain is due to leave the EU on March 29, a deal is 95 percent done but officials have repeatedly cautioned they are still haggling over the fate of the land border between British-ruled Northern Ireland and EU member..