Going after DNS resolvers seems like the easy win. If a website was breaking the law so egregiously then take it to ICANN to get the domain name seized. I'd wager that's a much harder thing to prove, hence the strong arming of DNS resolvers.
If the UK government wants to ban porn but loves gambling, while the US wants to ban gambling but loves porn, a blocking mechanism that lets them have different blocklists allows both nations to get the censorship their voters have chosen.