If we want to base the argument on technical nuance, 4chan are sending their packets to the U.K. just as the cocaine dealer would be sending packets (of cocaine) to their buyers in the U.K.
I would strongly disagree with that, in the sense of the layer of communication that 4chan operate at. I would argue that 4chan aren't sending packets to the UK any more than I'm currently sending my keystrokes to wherever you are reading this from - these actions are performed at a different layer.
If the UK wants to block packets from across the pond, they should (but I hope they don't) do it via a Great Firewall, rather than expecting random foreign websites to do it for them.
This isn't a physical product. A better analogy would be a phone call, initiated by someone in the UK to a foreign country.
The user mails you a box with a note that says "1kg of 4chan packets pls", and a prepaid return label to an address local to you. You put the packets in the box and kick it down the street to its "destination". Job done as far as you know.
The place you sent the box then repacks it and mails it to the UK. Somehow the UK thinks that you and only you have broken the law.
They're replying to an externally-established connection. The packets they're sending are going to a local router.
If you posted cocaine from your cocaine-legal country to an address where it was illegal, and you followed all the regular customs labelling rules, I'm not sure you should be liable. And you shouldn't be extradited either. Even the UK demands that extradition offences would have been criminal had they been committed in the UK. Now I'm sure in practice, you'd find yourself in trouble immediately but I don't think it's fair.
The ramifications of laws like this is everyone needs to be Geo-IP check every request, adhere to every local law. It's not the Internet we signed up for.