Hm, I'm going to need some software engineers to critique an idea I have that could at least partially solve the fears people have about their personal details being tied to their porn habits.
The system will be called the Adult Content Verification System (or Wank Card if you want to be funny). It's a physical card, printed by the government with a unique key printed on it. Those cards are then sold by any shop that has an alcohol license (premises or personal). You go in, show your ID to the clerk, buy the card. That card is proof that you're over 18, but it is not directly tied to you, you just have to be over 18 to buy it. The punishment for selling a Wank Card to someone under the age of 18 is the same as if you sold alcohol to someone under 18.
When you go to the porn site, they check if you're from the UK, they check if you have a key associated with your account. If not, they ask for one, you provide the key to the site, the site does an API call to https://wankcard.gov.uk/api/verify with the site's API key (freely generated) and the key on the card, gets a response saying "Yep! This is a valid key!" and hey presto, free to wank and nobody knows it's you! If you don't have an account, the verification would have to be tied to a cookie or something that disappears after a while for all you anonymous people.
As a result, you can both prove that you're over 18 (because you have the card) and some company over in San Francisco doesn't get your personal data, because you never actually record it anywhere. All you have is keys, and while yes, the government could record "Oh this key was used to verify on this site", they'd have to know which shop the key was bought from, who sold it, and who bought it, which is a lot more difficult to do unless the shopkeeper keeps records of everyone he's ever sold to.
So... Good idea? Bad idea? Better than the current approach anyway, I think.