Since the provisions of the Licensing Act 2003 came into force, there is no longer a minimum age for serving alcohol. However, each individual sale by a minor has to be supervised and approved by a licence holder (or by another adult authorised to supervise sales by a minor).
The situation is similar to that in your local supermarket. If the check-out operator is under 18 years old, he/she has to ask permission from an adult colleague, every time he/she serves alcohol. (The manager can't give 'blanket' authorisation for a minor to sell alcohol. The minor must obtain separate permission for each individual sale). The same applies to sales from a bar.
As stated, there is no minimum age defined by the Licensing Act 2003. However, it's illegal to employ any young person, in any job, if they're under 13 years of age. 13 year olds may only work in specific occupations approved by their local education authority. This never includes bar work and therefore explains why you were quoted a minimum age of 14 years.
Chris