If a country has media or broadcast standards laws, and you distribute or broadcast content in that country that violates those laws, that’s on you. The country can just fine you if you chose not to comply. Just the same as they would if you were doing it while living in that country. You’re not obliged to care about the fine if you don’t live there and never intend to travel there. But if you do then you’re going to be subject to their laws at that point, for violating those laws when you distributed that content in that country.
The hardware that propagates the data transmission is owned partly by the UK and partly by Canada. The Canadian website operator has turned off the transmission to the UK on their side and has fulfilled their obligations. The UK is complaining that they didn't turn off transmission on their side.
What you're saying is that the website operator should travel to the UK to enforce UK law from Canada. It's nonsensical.
Edit: If this wasn't clear enough here is a cartoonish version:
Ofcom: Your site violates UK law. By allowing UK citizens access, you must abide by UK law.
Website operator: I do not care about serving UK citizens and am now blocking UK IP addresses. Thank you for notifying us.
Ofcom: We have decided that we will not block access to your website from the UK. Therefore it is theoretically possible to access your website anyway, which is a violation of UK law. No matter how much effort you spend on ensuring that UK citizens do not gain access to your website, we will make sure that there will always be a non zero possibility of violating UK law. Since we are not blocking anything, the blame cannot lie in UK users circumventing a UK side block, which would force us to prosecute UK citizens rather than you as the website operator.
Please shut your website down to ensure compliance.
Website Operator: Okay so you're telling me I have to build the great firewall in the UK, make all ISPs adopt it and lobby a change in UK law to make the firewall mandatory, just so I can host my website?
Ofcom: yes
Wait did 4chan actually block UK addresses? My understanding was it hadn’t which makes your story fall apart.
The idea that a router is responsible for the packets it forwards rather than the person that made the content and put that content in those packets is getting silly.
It's fucking stupid that an American site that is afforded free speech protection in its own country has to deal with the UK acting like a tyrant.
There are so many global laws that are actually enforced. Of course they all origin in the US. See KYC/AML laws.