I think it was the drip drip drip of anti EU feeling, over many years, from the MSM that swung Brexit.
There is a telling snippet from Fawlty Towers, where telling the Major we don't shoot Germans, Basil says something like "didn't vote for it myself, but now we're in , we should make it work" (or words to that effect).
Growing up in the 70s, through the 80s, that was very much the attitude I experienced. Never met anyone who was fundamentally so against it they would cut their arm of for it.
Still, having discovered how much they can sway the electorate around an issue, the future looks fun. ECHR next. Then womens rights. Workers rights. Creation of the new aristocracy. All things we can expect to see.