Once you're in another country's jurisdiction, all bets are off. You're subject to those laws, unless there's a treaty or similar saying that you're not. In another post, I mentioned writing nasty blog posts about Kim Jong-un. If you do that, it's probably a very bad idea to visit North Korea.
In this case, the operator of 4Chan is free to blow off the UK's law. They may wish to account for that in future travel plans, though.
