Somewhere, an MI5 operative is quietly weeping into his beer
Somewhere, some of Britain's cadre of spooks who are charged with observing the Internet and reporting on possibly dodgy users thereon will be quietly mourning the forthcoming loss of a really good source of information. The thing is, Adult Verification (AV) tests are only fit for purpose if they can access Government-approved identity documents, so that the age of the person can be definitively verified. Once an AV system has established that User Fred was born at least 18 years ago, it isn't going to forget the ID documents that it saw.
So, if you as a consumer of adult material use Government AV, then there is a strong potential that someone somewhere can cross-reference what you looked at with definitive proof of who you are. Given the startlingly and abysmally poor standards of handling secret data that the Civil Service often shows, only a complete idiot is going to hand over documentation to this new scheme lest they be blackmailed at some future date.
No, the average person is going to buy a VPN connection which terminates somewhere outside the UK and use this for their browsing instead. The number of VPNs is thus going explode virtually overnight, and the utility of knowing that someone is trying to hide who they are from the Government drops dramatically.
Right about now, the main users of VPNs are businesses, remote workers, a few copyright pirates and a very few paranoids and loons, plus the odd terrorist or two.
As soon as the law is enforced, this select grouping of possibly-interesting users will be swamped with a tidal wave of, err, one-handed typists, shall we say. MI5 will lose a useful source of info from this little bit of stupidity.