It would depend on the type of job. I think that you have to be 13 to do a paper round, but if working in a shop the maximum hours you may work if over 15 is 8 hours, and 5 hours if under 15. Many shops though do not employ people under the age of 18, even if they do not sell alcohol. When I worked for the Warner Brothers Store they did not employ anyone under the age of 18.
In Lancashire you are supposed to have an employment licence if working whilst still in school, the forms for this are available in school. I don't know if it's the same in other areas.