I browsed through an article today where it was stated that U.K. Foreign Secretary Hammond said he would like vote to leave the E.U. This made me wonder: what exactly is the U.K.'s relationship with the E.U. I know that it is a member, but IIRC their relationship with the E.U. isn't the same as other nations. For example, in the UK they still use pounds instead of Euros. When did the U.K. join the E.U. and why? And can a nation just vote to leave the E.U. and it be accepted by the E.U. and other nations?