The UK was taken into the Common Market by the Conservatives led by Ted Heath in 1973, following a change of Government in the United Kingdom later that Year, the Labour Party started Renegotiations of the UK’s conditions of entry into the EU and called for a referendum on remaining in the Common Market, the outcome of the British referendum revealed that 67.2% of voters were in favour of the United Kingdom remaining a member of the Community, and the UK retained membership.
It's a urban myth that the UK population at that time didn't want to be part of the EU, after all, it was Winston Churchill's original Idea to have a "A kind of United States of Europe" back in 1946.