i'm a law student. the law on alcohol in the UK is actually more complicated than you think. believe it or not, you can actually legally drink at 5, yes 5. you are not allowed into a bar, or to buy alcohol from an off licence till your 18, but restaurants and other places that are not 'primarily for the sale of alcoholic beverages' you are allowed to drink alcohol provided someone over 18 buys it for you.
you can order alcohol yourself at 14. this is provided you have it with food and are with your parents.
you can order it with food but without your parents at 16.
the full drinking age is 18.
supermarkets sell all types of alcohol, theres no limit on percentage. grocery stores also sell alcohol, but they have to have a licence (hence 'off licence'), and spirits and stuff have to be sold behind the counter (cant be on the shelf).