Ok I'm not a fan of that stuff, but it's also not even close to the worst stuff out there. It's also a slippery slope to ban something that is inherently no more dangerous than standard porn. They're not actually related, not even by law, it's just (very poor) acting. If nothing else, there's a lot more important things going on than this.

Why is the UK like this?

The Ministry of Justice said that possessing and publishing porn showing incest between family members and sex between step or foster relatives where one person pretends to be under-18 would be a crime.

This is the key part of the law, and I think this was already illegal in the UK, so I'm not entirely sure what has changed here.