This is the right framing. No site, including 4chan, is forcing their content on innocent Brits. The only way people in the UK see 4chan is by proactively establishing a connection to the site and requesting the download of data. Those users, not 4chan, are the active agents. If the UK government wants to control what its subjects request online, they should pass laws regulating that behavior.
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".
Two counter points. First, crypto is not part of the UK government's financial systems or institutions. They don't automatically hold jurisdiction over all crypto transactions, or more specifically, crypto service providers.
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.
Just because UK internet users are able to establish a network connection to 4chan’s server via ISP peering agreements does not mean 4chan are subject to UK law.