Entwisti was right actually, asda was actually one of the first companies to stop selling tobacco to under 18's before the law was actually finalised, at least in my local asda anyway, and has the ability to do the same with alcohol even if it isn't actually written in law that thats policy. Policy doesn't have to be formally written in law, thats why its policy.
Also his explanation of the law is spot on aswell and the way he explains it with the example of the speed limit is good aswell.
If ASDA dont want to sell you something they dont have to, and that stands for any product whether its got age restrictions or not. Different stores might have different policies on it and sell to over 18's or just over 21's, but both are legal and it is not making their own laws, it comes under the law of contract.