It's enforced in a bizarre way in Waitrose.
I have registered to scan my own shopping at Waitrose - it saves a lot of time because you can bag up your shopping as you go round the store. To be able to do this you have to have an account card or credit card from John Lewis or Waitrose, so your age is in their system. There's never any hold-up buying alcohol, for example.
Today I was ready to pay when the check-out machine flashed up "seek assistance". Turns out it was because I was buying the Sunday Times, which had a DVD inside. So according to Waitrose, I'm old enough to buy alcohol but not to buy the Sunday Times. The DVD was a 15 certificate.