Actually it’s the opposite, the UK was in an economic slump in the 60s when it was part of the efta, the European free trade association with Norway etc. But the “ever closer political union “ EC was booming economically so the UK begged to join, THRICE.
It was rejected twice by de Gaulle but got in in 1972.
The UK dumped it’s free trade partners to join the ever closer political union.
Its economy then boomed.
It was booming also until the recession
Is the EU still a benefit to the UK or a hindrance? Bureaucracies start out small and with good intentions and devolve into all consuming monstrosities.