I've suggested numerous times one of the easier ways to address this and split the responsibility and liability evenly among parties. [1] Most importantly, this process needs to be overhauled to keep PII out of the age-check websites. Such websites will be a very tantalizing target for attackers to mass gather credit card numbers.
In my opinion, the UK government is setting itself up for a lot of political blow-back when that site is compromised.
[1] - https://news.ycombinator.com/item?id=19685800