That’s not how the internet works. They operate a website that is exposed on the internet. The key here is exposed. They aren’t coming over to the UK to operate. When you visit their site you’re going to them.
If they don't comply, the UK will order ISPs to block access.
If 4chan really genuinely doesn't care about UK users ("it's not our fault some people from the UK access our site"), then they won't be bothered about this. If the loss of UK users is considered significant to them, then clearly they are actively interested in serving the UK market.
It’s like saying a small village in France that has a bakery serving cum filled doughnuts (LOL I don’t know WTF French people like to do) serves visitors in the UK because there is a railway line in the village connected to Eurostar which in turn connects to the UK. At that point they are only serving visitors that happen to be in that little French village. And if no French laws against cum filled doughnuts exist then why should the bakery pay UK fines?
What matters is where the customer is at the time, not where they're from. If a British person goes to France and buys food from a French shop, they are for all intents and purposes a French customer at the time the sale takes place; French food standards laws would apply.
By the same token, if you travel to France and then use the internet to access 4chan, you are a French internet user for the purposes of that interaction, and French laws would apply to you. If a French person came to the UK and accessed 4chan, British laws would apply to them at that time.
Now if your hypothetical French bakery wanted to export its products to the UK (i.e. sell them to British consumers who are in Britain), they'd need to meet British food standards. The fact that they're a French business doesn't exempt them from that.