As 4Chan has no assets in the United Kingdom (given that it has no connection to the United Kingdom), that would require you to show up in a US court as a platiff

I think that the way this works is that first the British government has to go to a British court and secure a ruling against 4Chan and then go to a US court and prove to it that 4Chan is under British jurisdiction to get that ruling enforced by a US court. As an aside, I think I'd be careful if I were 4Chan

there are things that they can do that are not immediately obvious, like targeted advertising aimed at Brits, that might place them under British jurisdiction in the eyes of US courts. I also dunno about accepting donations from Brits. But if they rigorously kept their nose clean, then I agree

it's very much possible that a US court would rule that there is no British jurisdiction.

However, I do have to say that I think that the prospect of Ofcom arguing the matter in front of a serious-looking judge in a British court, horsehair wig and all, while presenting material from, say, /d/ (NSFW), and having said judge gravely contemplate it and British law seems like it'd be sound fodder for a comedy skit.