If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in United Kingdom.
Under the BBPA's Challenge 21 and Challenge 25 schemes, customers attempting to buy alcohol beverages are asked to prove their age if in the retailer's opinion they look under 21 (or optionally 25) even though the law states they must be a minimum of 18. Many supermarket and off-licence chains display Challenge 21 (or Challenge 25) notices stating that they will not serve people who look under 21 (or 25) without ID.
You can find more legal details and links here:
Wikipedia Legal Drinking Page