If the UK is serious, they could ban entry to the 4chan owner(s) or even arrest them upon entry, so it's not something to just ignore or laugh off.
Publically available databases suggest 4chan executives include John Cena, Evan Essence and Norton Antivenom.
And with a bit of effort, the UK could call on friendly countries for extradition (such as Canada). They'll have to convince the judge that the crime is bad enough for extradition, but I'm sure they can come up with something.
It all depends on how much the UK really cares. In practice, this is probably just a big act by Ofcom so they can say "we've tried everything, we'll have to go for a full ISP-level block as a last resort".