@cstross @Fonant
Unless I misunderstood the proposed law, it's the VPN operator that would be prosecuted in this case. They may lose their ability to take money from people in the UK.
If I'm over 18, it is not illegal for me to use the VPN, so someone would have to prove that I am using it but no one checked that I was over 18. If I am under 18, then the provider is at more legal risk but they could claim that they did age verification and this user managed to bypass it somehow.
The simplest way of doing age verification is to require a payment from a credit card in your name. The easiest way of bypassing this is to use a parent's credit card. If a company takes payment for VPN use via credit card, and makes a minimal effort to not accept debit cards or pre-paid cards for folks in the UK, they're probably okay.
Which doesn't mean that this is in any way a sensible law.