You think UK would be the first country to ban VPNs? There are thousands of talented and very committed computer scientists in authoritarian countries tirelessly working to enforce internet censorship. They discovered many wonderful technical solutions to this problem.

All the mainstream VPN protocols like OpenVPN and Wireguard don't even try to hide themselves and are easily detected no matter what port you use. They are useless if you country is seriously set to block VPNs.

There are different protocols specifically designed to circumvent censorship and they do so by masking their traffic to like something innocuous like HTTPS. However even they can be detected using advanced traffic analysis. For example, if a given machine only sends and receives HTTPS traffic to a single specific overseas server, it is safe to assume that it's not actually a genuine website traffic but a VPN masquerading as HTTPS.

There is special hardware that all users' traffic goes through that detects these patterns and automatically throttles/blocks these connections.

Presumably they'd learn the IP addresses the VPN providers are using and watch for connections to those.

China manages to figure it out using deep packet inspection.

Funnily enough, they don't fully ban them. Just randomly cut them off to inconvenience the user.

Most of them do that too a degree anyway. Credit cards are only available to over 18s, so if you pay with one of those, that's your ID. I guess this is mostly for free VPNs (although I've never found one that was any good) and people paying with debit cards or crypto.