The real purpose of the E.U. is not economic, but political. It began as the ECSC, which was geared to making sure that Germany would not re-militarize by extracting iron from the Rhine region. The purpose of the E.U. is to obviate the sort of bloodshed that Britain saw in WWI and WWII. If the British people don't want to be in the E.U., then you should leave. I don't believe that even your own government should keep you from deciding such a matter as a people, directly. That said, with great power comes great responsibility, and this applies to popular sovereignty. In other words, the people taking up the mantle of direct democracy in a constitutional referendum should make an informed decision, looking beyond even the people's own immediate interests. The stakes are much, much higher than whether being in the E.U. is an economic net loss or gain to Britain on a yearly basis, or even whether the City is crimped or inconvenienced. Much more is at stake.
From: "Should Britain Secede from the E.U.?"
