It explicitly says that 7% of their users are coming from UK. If UK blocks them, they will loose noticeable part of advertising revenue. If there was no money at stake, they could just ignore Ofcom and sleep well. But they appear to be very agitated about the fact that they may loose their second biggest market.
Honestly, I don’t understand anyone on 4chan side here: they are de facto in UK jurisdiction because they earn money from that user base, so either they comply or they leave. All of this freedom-of-speech and US lawsuit hype is just a distraction circus.