Persons aged 12 and over can have a meal with beer, cider or wine in a restaurant or in a pub with an area specifically set aside for meals, providing it is eaten with a person aged over 18 present. Liqueur confectionery sale and consumption age is 16. Otherwise the minimum purchase and drinking age in "relevant premises" is 18. http://www.erowid.org/chemicals/alcohol/alcoho l_law2.shtml
some places won't sell to 18 year olds even with ID as my son will testify, he tried to buy some last weekend at a petrol station and was refused as they only sell to over 21's
Jules001 - I think the garage got the wrong end of the stick on the new government ID campaign there! They encourage people to ask for ID if the person appears under 21, but they only need to prove they're 18.