Plowter is out of date regarding minors selling alcohol. (The rules changed with the 2003 Licensing Act).
As Sammy states, minors can serve alcohol (both in shops and in pubs) as long as each sale is individually approved by the licensee (or by an adult to whom the licensee has given permission to approve such sales). That's why, in stores like Asda and Tesco, you'll see young check-out operators seeking permission for alcohol sales from adult collegues. As stated, every sale has to be specifically authorised. The licensee can't give blanket permission for a young person to sell alcohol.
However, it's illegal to employ any young person, of school age, without a permit from the local education authority. A permit will only be granted if the LEA is satisfied that the work is of a suitable nature and meets the statutory requirements regarding the hours worked.
A 15-year old may not work more than 12 hours per week during term time. She is limited to a maximum of 2 hours per day on school days. She may not work after 7.00pm, even during school holidays:
http://www.direct.gov.uk/en/Parents/FamilyIssu esAndTheLaw/ChildrensRights/DG_4002945
Breaches of the rules, regarding the sales of cigarettes and alcohol by minors, should be reported to your local Trading Standards department. (Unless you live in an area under a unitary authority, look for the phone number under the name of your county council, not the the district council).
Breaches of the rules regarding child employment should be referred to the local LEA. (Once again, the phone number will be listed under the name of your county council or unitary authority. Ask for the 'educational welfare' department).
Chris