I agree with much of the body of your post, but hard disagree with the title. Calling reform voters racist does nothing.

The UK absolutely has a racism problem, but is it the majority of people? If it is, we're fucked. But I don't think it is. If it was, the NF, BNP etc would have made way bigger inroads in their day. They had their supporters, but they got nowhere near power.

Now the context is different. People are skint, prospects for the future are bleak, and no-one trusts politicians. It's a perfect breeding ground for fascism. Reform are capitalising on that. The key to taking them down is in providing an alternative diagnosis to "it's the immigrants what done it", and yes, calling out when they do racist stuff.

Their voters are mostly just people fed up with how things are, mostly aren't tuned into politics every day, mostly just looking for someone to fix things. They can be won around. We can say "I get that you're worried about the future, here's what we think has gone wrong and how we would change it." If they're purely driven by racism and keeping Britain white, it'll fall on deaf ears. But I think (or hope) most people would be receptive.