Danny's post is spot on. (That's hardly surprising. If I recall previous posts correctly, Danny is a solicitor and licensing is his specialist field).
The only purpose of my post is to stress that it's only legal for a person under 18 to serve alcohol in a bar if each individual sale is authorised by an appropriate person. This is the same as the situation in a supermarket where young check-out operators have to seek the permission of an adult colleague every time someone buys alcohol. The supermarket manager can't give 'blanket permission' for a young person to sell alcohol and neither can a bar manager. So, although a person under 18 can theoretically serve alcohol, in practice nobody under 18 is likely to be employed behind a bar because no bar manager would want to have to authorise every single sale on a busy Saturday night.
Chris