"Offering to" is a nonsense term. The website exists on the open internet regardless of jurisdictional borders. A UK citizen must actively go to the website, initiating a connection from within the UK and requesting data from an IP address that may or may not have some kind of relationship with geography.

4Chan isn't popping up unbidden on people's phones. Wither a UK citizen chooses to visit a website is no business of the website operator.

To say that 4Chan is somehow responsible for the actions of unknowably many private citizens is absurd. If the UK wants to enforce internet censorship within their borders, that's their own business. Putting pressure on wholly independent foreign businesses for the crime of existing is not reasonable. This is just as bad as US credit card companies censoring adult material from the entire global online economy.

They're trying to censor large parts of the global internet for everyone, not just their citizens. If they cared about UK citizens so much, they'd do something like proactively blocking noncompliant websites to force them to immediately either comply or fuck off. They should be trying to protect their citizens instead of trying to bully foreign companies they have no jurisdiction over. It's their responsibility to enforce their laws, not the US courts.