The UK does not have jurisdictional power over anything outside their country - they can not a foreign site to do age verification of foreign residents.
Now, the UK can say that they need to check for all UK residents, regardless of them using VPNs. But if there are no practical way to do this, I think the UK will have diplomatic issues enforcing anything to non UK companies breaking that laws - as they would need, eg. Germany, to help them enforcing the law on certain providers.