In the UK, ~55% of traffic comes from mobile [1]. The UK could approach Apple and Google and ask them to remove VPNs from their respective app stores when opened in the UK.
I imagine this would curtail a large proportion of mobile VPN usage.
Blocking desktop VPNs would be a bit more adhoc but it is possible to make it much harder for many people to download VPN clients.
Watching sports for cheap using a "dodgy firestick" or similar hardware is incredibly popular in the UK to watch sports for cheap even among those not tech-inclined, despite obviously being illegal. I'd predict the same to happen here, quickly you'll see plug-and-play boxes that will route their home wi-fi traffic through a VPN, and most people will have a mate of a mate who sells them for a few tenners.
Deep packet inspection can detect VPNs. The problem may be more that people have legitimate uses for VPNs, like at their work. Those could be whitelisted though.
I am pretty sure we are slowly but surely heading towards a point where every country will implement its own great firewall and block every website except those in a whitelist approved by the government.
The mechanism for China to compel Starlink to not operate there seems more clear: the leader decides to block all direct and indirect business activities in China, related to Elon Musk.
Is that comparable, for the UK? Maybe other EU members would agree?
As an American, I have trouble comprehending multiple governments colluding to enable mass government censorship, and expecting to stay in power. But, I know nothing of the European mindset.
I imagine this would curtail a large proportion of mobile VPN usage.
Blocking desktop VPNs would be a bit more adhoc but it is possible to make it much harder for many people to download VPN clients.
[1] https://www.digitalsilk.com/digital-trends/mobile-vs-desktop...