https://news.ycombinator.com/item?id=47442838
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.