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.

[1] https://www.digitalsilk.com/digital-trends/mobile-vs-desktop...