It isn't true though.
The UK was never truly part of the European Union
Here is a part of the article. You will find the same thing said all over.
Remainers conveniently forget that when Britain voted in 1975 to remain a member of the EEC — after joining in 1973 — the referendum was based on the lie that membership had no political implications. In fact, the EU’s founders, especially Jean Monnet, saw ever-deeper economic union as a way to forge ever-deeper political union. In 1986, Thatcher signed the Single European Act (which set the objective of establishing a single market), apparently believing that it was only an extension of free trade in goods to services, capital, and labour. But Britain’s semi-detached status was confirmed by the Maastricht Treaty of 1992, under which Thatcher’s successor, John Major, obtained (together with Denmark) an exemption from the requirement to join the euro. More obviously than anything preceding it, the single currency was a touchstone of willingness to proceed toward political union. After all, as the events of 2008-09 showed, a common currency without a common government cannot be made to work.