Except this can only be fair if they carded everyone who buys liquor, not only people who appear young, otherwise it's subjective, and businesses shouldn't be liable if a tall, bearded teen buys vodka, because he looks older than 18.
Of course, in reality, liquor store cashiers are allowed to judge subjectively, but VPN providers won't be allowed to. And they'll probably be asked to share records of registered adults in the future, given the repeated efforts to backdoor encryption in the same UK. This is unlikely to be only about protecting the children.