but ISPs do not have the power to decide if something is infringing a rule or not.
Thus Ofcom goes through the motions of telling 4Chan "hey I think you're not compliant" and if 4Chan says "lol, I'm not serving UK people" _then_ the UK authority will tell the ISPs to block it (and the will be on the hook if they don't).
If I open a service to pick up stuff from whatever US shop a UK customer wants and then ship it to him would I not be on the hook for the contents? I don't think customs would be impressed with the argument that I don't have the power to inspect sealed packages handed over to me.
Thus Ofcom goes through the motions of telling 4Chan "hey I think you're not compliant" and if 4Chan says "lol, I'm not serving UK people" _then_ the UK authority will tell the ISPs to block it (and the will be on the hook if they don't).