A victory for Prime Minister David Cameron's Conservative Party could bring Britain a step closer to leaving the 28-nation European Union, a move many companies fear would cause damage by increasing costs of doing business on the continent and eroding London's role as a global financial hub. "Leaving the EU would be a disaster," he said in the offices of Kinson Capital Ltd., which manages assets and provides advisory services for banks and hedge funds. Cameron promised to hold a referendum on EU membership in 2017 as he sought to blunt the rise of the U.K.