I'm a bit out of the loop and not familiar with the whole thing. Do you just submit a plausibly real name and any DOB that is older than 18 and a picture? Does the thing crosscheck the name + dob with a demographic database (does UK has any)?

The Online Safety Act is completely stupid - it just requires "age verification". Various sites have outsourced this to various providers, most of which are American. The age verifiers in turn just look at the image, generally algorithmically without human intervention, and go "seems legit". There is no single national UK ID database that could be used for this purpose - everything is either partial, doesn't include photos (e.g. electoral roll), or isn't public (passport DB).

It's all just random third party shit with zero government oversight. There's no national ID in the UK because even the mildest mention of a voluntary national ID scheme leads to gnashing of teeth and wailing about how it's fascist and a violation of our civil liberties and everyone is happy pissing about with driving licenses or passports or utility bills to prove who you are.