If the vpn endpoint is in Rome or New York City, how will the UK government force that non-British vpn service and that non-British porn site to verify the age of anyone using it?
It's easy enough to get a list of IP addresses from those vpn services and just block them if you're Netflix, but to force compliance on anyone traversing the tunnel is another thing entirely. The UK government would have an easier time banning vpns outright.
These can be wildly effective at such matters. I'm sure most countries can come to some understanding with the UK on the matter; be that foreign aid, trade concessions, assistance with their own law enforcement, or perhaps acknowledgement/support on the international stage.