There's a belief that one of the original driving forces of the raison d'être of the Union was to bind countries so close together that war, an endemic disease in Europe, might be rendered impossible.
Were it not for the African/mid-eastern problems of migration, I am sure non of this shit would have gone down. In the street, you can't tell a Brit from a Pole from a German (should the three be blondes) but the ones from outwith Europe are easy to spot. The UK problems started way before the Common Market was even a cold, wet dream down in somebody's pyjamas. It began with Empire, cats, just as in the US you had slavery. Son pays for Dad's mistakes. It's biblical and visibly true.
That becomes the most easy and convenient trigger to touch when you want to find something to advance your personal political ambitions. And boy, did some of those bastards lie! And worse, few of their followers seem to care, even though they probably know the inconvenient truth. The power of thought does not imply that folks also think.