If the UK is serious about blocking VPNs that don't comply they'll mostly succeed for the big ones. They'll get them removed from app stores which will prevent most normies from finding and using them. They'll apply network blocks to their entrance IP addresses (laughably easy, there are commercial vendors who sell data like this so they don't even need to invent the wheel here) and make it difficult. They wouldn't be able to prevent truly determined VPN providers from providing service but the days of $4/month for privacy/torrenting would be gone as the prices would likely be higher and you'd have to do things like mail cash.

Beyond the known IPs, VPN traffic is fairly easy to flag with DPI solutions and could be detected and blocked or dropped by ISPs acting under the law. This could also be used to stop people running tunnels to hosted VPS solutions outside of the country or run by friends from their homes. There are obviously ways around these, disguising traffic, various techniques but for most people they'd give up and either stop browsing porn or cough up their ID. Of course this would create a dangerous state of affairs where anyone using a VPN without being KYC'ed is clearly a criminal, at the very least a suspected video pirate, at the most a dangerous child predator or terrorist.

Additionally the UK isn't like Russia or China, lots of western CEOs and employees pass through and within its jurisdictions and if a particular VPN is providing service without this they could try and arrest c-suite people or engineering staff associated with it and slam them with jail time. So that's a problem.