The British joined to screw the French by splitting them off from the Germans that and of course their foreign policy objective for at least the last five hundred years which has being to create a disunited Europe.
For anyone else interested, the parent paraphrased a scene [0] from the legendary British sitcom "Yes, Minister" (~early 80s) where the civil servant Humphrey explains, in the most cynical manner possible, how the EU (at the time EEC) came to be.
Its entry was personally blocked 3 times by Charles De Gaulle before finally being allowed to join in 1973.
When the UK finally left in 2020 it had been a member for 47 years.
I wouldn't describe that as coming late and leaving promptly.
As for never buying the EU project, a 1975 referendum on EEC membership was won by a comfortable 34.4% majority.
Where as the 2016 vote leave campaigns won with just a 3.7% majority, having broken the rules on spending.
I would say that many of us in the UK buy the EU project and are unhappy about the leave result still.
To this day the reports on foreign interference have not been compiled or fully published by the current Conservative government.