Am I missing something here? 4chan is available in the UK so has to follow UK laws there, where is the problem? Regardless of whatever it is they are enforcing.
Suppose North Korea sends you a letter demanding that you take down a blog post joking about Kim Jong-un being chubby, because that's illegal in North Korea. Do you feel obligated to comply with that demand? After all, your blog could possibly be read by someone in North Korea.
I don't have anything against the UK. They've been our good buddies since a spat we had a couple hundred years ago. But I feel every bit as obligated to follow UK law as to obey North Korean law, which is to say, not at all.
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.
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.