Under the Licensing Act 2003 a person under 18 is allowed to sell or supply alcohol which is being served with a sit-down meal in a premises which is only used for the service of sit-down meals.
For sales across a bar, however, every sale or supply of alcohol by someone under 18 must be specifically approved by the designated premises supervisor (DPS), the premises licence holder or someone over 18 who has been authorised by either the DPS or the premises licence holder. If the sale or supply has not been specifically approved then it is the person who knowingly allows the sale or supply to take place who is guilty of an offence.
In practice this means that someone under 18 may work in a pub, but someone responsible in that pub must specifically approve every sale or supply of alcohol that they make. Think of the example of a supermarket where under 18's work on the checkouts, but must have a supervisor come to their till and specifically approve each sale of alcohol.
Interestingly the 2003 Act did away with a the lower age limit for serving alcohol so you got this interesting situation
http://www.dailymail.co.uk/news/article-419409 /Meet-12-year-old-pub-landlord.html