Speaking as a UK citizen: you're exactly right. If the UK wants to prevent 4chan from being imported into the UK then it needs to block it at the border as it would for physical goods.
The fact that's technically hard to do (at least without going full-on CCP) doesn't change the situation. Attempting to fine a foreign entity for doing something that breaks no laws in the foreign entity's jurisdiction is just risible.
It's very much a rock-and-a-hard-place situation. "It's an import", so they have to respond to it like they'd respond to imports...
But unlike physical imports, there's a sense that blocking these imports is an affront to base philosophical freedom in a way that prohibiting physical imports isn't.
I hope they do block it.
And we shall call it "the Great Firewall of the UK".
It is amazing that these guys don't see the irony of monkeying totaliterian states policies, in term of surveillance and censorship.