The VPN trick potentially won’t last long. We’ve seen it go stale already in the world of intellectual property rights. For at least the last ten years Netflix et al have been well aware of which AS numbers / IP netblocks correspond to people sat at home in front of the TV, and which correspond to servers in a rack somewhere (including those hosting VPN endpoints.) One tweak to the rules and all of a sudden not only…
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.