A vote will make the government look weak. The government will seem like it's avoiding a difficult issue by shifting responsibility for the Europe question to the public. Europeans will see the British government as an unreliable political partner willing to gamble EU membership at a volatile and dangerous time for the continent's economic and political future. To lose the trust and co-operation of Europe by permitting a referendum would be myopic at best, and replace long-term political co-operation and security with short-term appeasement of the general public.