Yes, way too tight! I was in Sainsbury's last night and even though the legal age is 18 they say you have to be 21 or over to buy alcohol! They even stopped a mate of mine buying a bottle of Whisky for his Dad's birthday because he was 20, a fact which his ID backed up.
Not only that, they now won't sell you DVD's unless your 21 or over too.
Haven't read the whole thread so some/all of what I am about to say may have already come up.
Azagoth, in your case you just got someone who didn't understand what Think 21 is. I have had to buy beer on a number of occassions for my brother, even though he has ID. Sainsbury's is making a big deal out of Think 21 now as not only do they realise the confusion, but the police and trading standards are getting very strict. They are even planning secret checks on stores in the north to see if they are up to scratch.
The reason why stores can seem very strict, especially when you have no ID, is because of the consequences which are pretty steep. If someone is found to have sold alcohol to someone under the age of 18 a combination of the following will happen:
Colleague in question either gets on the spot £80 fine or a police caution.
Colleague in question could also be prosecuted.
Store manager can be prosecuted and fined up to £10,000.
The store can lose its license, and will lose the right to sell any alcohol.
The company can be fined lots of money too.
As you can see, it is usually better to have someone slightly annoyed at you than being the cause of lots of bother to yourself, the manager and the store.
So as has been said, carry ID. Simple as. If you still get refused, ask for a manager, as not everyone behind a till has a full deck.