If they want their assets, they will have to use U.S courts to get them and U.S courts will refuse to enforce British law that violates the first amendment. It's pretty simple actually. If they had assets in Britain, then they could get to them, but they don't.
They can just treat 4chan as malware server or a drug cartel. There exist sanction mechanisms against foreign entities that do not use local law enforcement in which case opinion of US courts will be irrelevant.
