They should never have gotten involved in the EU boondoggle to begin with. The UK has never had a common language or culture with the rest of continental Europe, and trying to shoehorn themselves into a Union was foolish.
Neither has the rest of continental Europe.
To some degree, it’s not the notion of some kind of union, but its nature. Montesquieu would have called for tearing down the EU as it formed from its beginning, due to the fact that it was against liberty (having both executive and legislative power in the hands of an unelected body of magistrates).