Ofcom does know that they're dealing with 4chan, right?

Ultimately all of these sorts of regulations rely on people feeling the need to comply. 4chan feels no needs, least of all to comply.

It's the immovable object of online forums. It has not encountered a true unstoppable force. I doubt it ever will.

If they want it "gone" they'll have to both block it at the infrastructure level leading into the country and keep people from using internet infrastructure that isn't subject to these blocks from within the UK. That's... not really possible.