This Identity thing is the key
The basic problem with any system which requires the user to prove their age is that this requires exposing identity documentation to an unverified third party. However many layers of obfuscation are put in, this problem remains: someone has a list of identities, someone else has a list of what they've been looking at.
Given the unenviable record that civil servants have of bypassing privacy checks and carelessly losing data, this ought to worry any sane person. I'd give it three months before a dataset linking names to porn viewed is accidentally left on a train somewhere.
A much easier way around this problem is to pay for a VPN service. The only info exposed to the VPN supplier will be payment info, and URLs visited; not content, not identities, and not anything really damaging. As the reputation and business model of a VPN relies on secrecy, many simply do not log this data, meaning that what they don't have cannot be leaked.
That's the light side. The dark side is what happens when a teenager who has not got any way to pay for anything online tries looking for porn. They are the group most hormonally motivated to want to see this material, and the group least intellectually able to choose a safe means of doing so. They are the people who'll use free VPNs, and thus the people whose every online move will be tracked by the mostly Chinese-owned free VPNs.
This raises the spectre of kids seeing exactly the same imagery that they got before, but having all this behaviour tracked by criminals who can then use it as extortion material. In other words, nice one UK Law-makers; you made a situation much, much worse through your actions.