The Brits joined later because they were broke and had lost their empire.
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.