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.
>4chan is showing ads on their site, but if your idea had any grounds, the issue would be with the ad network, not 4chan.
It's hard to understand the logic of this statement. Why the ad network? 4chan business is to show ads to users while offering them a platform for conversations. What 3rd party service do they use is irrelevant unless that is by coincidence an UK company.
>More realistically, 4chan will likely be banned by UK ISPs after a court ruling.