I'm not sure why they should care. Just banhammer the entire island. Everybody should. They can have their own independent internet, that conforms to UK laws. Let's see when HN will be fined, for advocacy of circumvention of local laws, or of strong encryption.
Facebook banned all Canadian news outlets, and they were probably actually making a few bucks from them. I can't imagine that 4chan would care too much about losing UK users. 4% of barely any revenue, maybe? Just ban them all; they're insignificant. The only drawback for UK citizens is that Reform will go up a point or two in the polls from people who prefer the authoritarianism they don't know to the authoritarianism they do, but considering the awful alternatives it's really six of one a half-dozen of the other.
One of the rational reasons people are preferring nationalists is simply because they are less powerful than the neolibs, who all work together against their populations. Better a the dumb local boss you know than a being the local outpost of a faceless world boss.