Banning in-store sales of VPN activation codes seems well within the ability of the British state to do, especially when they already banned bank/credit card payments.

Mullvad also let's you buy by mailing them cash. Or bank wire, or crypto.

Not on an age basis since we have decades of precedent for retail staff to judge age and not keep a copy of ID. It will have to be on a more blatantly 'we want to be more like the Chinese communist party' basis.