Ofcom is simply doing their job. I doubt they care about the users of 4chan. They will fine the company in accordance to UK law. Then if the company does not comply Ofcom will target their advertisers and it's Japanese owner who lives in France as well as having UK ISPs block 4chan. I can't think of any reason as to why France wouldn't work with UK authorities on this.
Contrary to HN and other USA tech forums might think, this will likely be recieved favorable by the the UK public.
For France to be legally able to give a shit Ofcom would need to go to court in the UK, pierce the corporate veil, and receive a final judgment against the owner of 4chan. Only then would they have some routes to petition French authorities for assistance.
There's no agreement between the UK and France that would require or even permit French authorities to enforce fines by a some random UK entity willy-nilly.
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.