Don't give up your day job.

The UK can block whatever they want if they'd like to become an authoritarian firewall state.

But they have no legal basis to fine 4chan.

I disagree. It's no different from selling to a foreign buyer by sending the product in the mail. You're not doing business in their country, and it's the buyer's responsibility to adhere to their local laws about imports, not yours.

4Chan has blocked the entire UK IP range. They do not host any infrastructure there.

They are bound by UK law exactly as much as they are bound by Venutian or Mars law.