I Believe it is 18!
But there maybe a loophole in the law!
I've been served at a supermarket before and the checkout 'kid' simply had to have it acknowledged by someone over 18 that he was selling it. Or so it seemed!
QPR has a point, under 18 just have to ask an over 18 in supermarkets - maybe thats part of the assisting in the sale of alcohol e.g. they are being supervised meaning they are then more like a waiter? as you can see i don't really know, under 18's do sell alcohol in all my local supermarkets