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.
I think I agree with this.
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.
Fair. But the UK law isn't going after the user it's going after the provider. In another country. That's the distinction, right?