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).