As long as 4chan sells 4chan passes to UK citizens, they do business in the UK. They sell using crypto so there's not much for the UK to go after, but they do more than just "be available".
Second, again, 4chan does not operate in the UK. If someone in the UK purchases a 4chan pass, they have electronically transmitted their "money" over to the US to buy it. I would compare this to a UK citizen flying over to the US and buying a ticket, and bringing it back with them to the UK.
It's very clear, 4chan did not perform any business or transactions within UK jurisdiction.