"There are very good reasons why successive [British] prime ministers of different political parties have always opposed a two-speed Europe. Once we are outside the crucial decision-taking bodies of the EU we lose all power and influence over our own future. (...) So the only real alternative to full and enthusiastic participation in Europe is departure, including from the single market, which is what the eurosceptics really want. This would not mean a return to imperial greatness, or the laughable idea of a new Commonwealth community. It would mean becoming a sort of Greater Cayman Islands, a tax haven and offshore shelter, as far as the other Europeans would tolerate it, with very little influence inside the continent or in the rest of the world. (...) If the prime minister [David Cameron] is to call a referendum in his speech, the only real choice is between being fully in or out altogether. There is no mythical third way."
Jonathan Powell, "Out of Europe, Britain faces a weak future" (Financial Times).