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.)
I don't believe this is the case, given that there is no legislation regarding this filter, meaning that there can't be a list of sites which must be blocked.
There's the issue of Cleanfeed and the IWF, but that's not a government thing.
EDIT: Obviously ISPs also decide whether to block something, but Govt. has a list of sites which must be blocked