In this case; the UK is reaching too far. Genuinely speaking; they don’t have the right to fine you if you don’t live or operate in that country. 4chan never did have any legal presence in the UK; even if it did accept ‘donations’ from UK citizens.

At worst; the UK can block 4chan from being accessed in their country and seize any money sent to 4chan by their citizenry in the future. I doubt anyone would care if that’s what they did.

The US specifically even states in it’s constitution that no citizen shall have laws imposed on them by another country that restrict their freedoms.