Teenagers have been looking at porn since forever. It's practically a trope of teens stealing their parents' porn mags. I don't think any of this has actually caused major societal issues.
The proposed solutions merely require that a teen steal their parent's identification, briefly, to create a porn account and move on. Heck, they can probably buy that information online if they are innovative enough. They certainly will be selling access to their porn accounts to their classmates. And even if they don't go through all that trouble, getting a porn mag is still pretty possible in the UK.
That makes this just a bad law. It doesn't meaningfully stop the problem it's meant to stop and it's expensive and intrusive. Even if privacy preserving age verification was bulletproof and perfect, you still have the access holes all over.
And then there's the simple fact that other nations exist. Yes, mainstream sites will put up protections, but what about the sealand porn site? Unless the UK wants a great firewall (ala the chinese firewall), they simply aren't going to stop this problem. Even then, VPNs are common knowledge at this point due to streaming.
Bad law, bad effects, and a pointless fight.