There's no evidence (yet) of there being ANY Government list of sites which must be blocked. All ISPs are going their own way and basically letting the decision

There's no evidence (yet) of there being ANY Government list of sites which must be blocked. All ISPs are going their own way and basically letting the decisions be made by whatever technology partner they have engaged to back-end the filtering. The only thing close to a UK-wide list of sites which MUST be blocked is the IWF/CEOP list, which exists specifically to deny access to images of child abuse and selected 1970s rock albums. (Search "Virgin Killer IWF") - That filtering is entirely separate and long-established in the UK, known as "Cleanfeed", and generally adopted by most major ISPs (but again not all.)