Ever since the United Kingdom joined the European Economic Community in 1973, after the French withdrew Charles de Gaulle's veto of its membership, Britain's relationship with the European integration process has been strained.
The British are reluctant Europeans, for historical and cultural reasons. For centuries, British foreign policy strove to avoid permanent European entanglements; but, most importantly, it aimed to prevent a single continental power from achieving dominance — especially if that power happened to be France. In the meantime, the British colonized large portions of the globe. Later, after the sun set on their empire, they tried to maintain a "special relationship" with the United States.
Joining the European Union was not an affirmation of belief in European integration, but rather a reluctant recognition that the trans-Atlantic strategy had run its course. British public opinion concerning the EU has since remained lukewarm, at best.