Living in Essex with a black wife and black in-laws, 15% seems fairly realistic. I think people would be genuinely shocked by the attitudes of a sizeable chunk of the population. I know I was.
Let’s be honest, if you're white, you haven’t really got a clue because you never see it or experience it. I was admittedly entirely ignorant to it myself before I met my wife and was firmly in the “racism isn’t really a thing in this country” camp.
I’m not claiming it’s worse than other countries, mind. It is definitely better than Portugal, probably Spain and Italy too. That doesn’t mean it’s not a thing.
That said, I actually do agree with you, there’s more behind the Reform vote than just simple racism. It’s a whole counter-culture that is basically nostalgia goggles for a segment of the population that thinks the British Empire was all really great and had no problems and we should go back to it.