Firstly, make sure that you know the rules which employers must observe. It's illegal for anyone to employ you for more than 5 hours on a Saturday, or for more than 2 hours on a Sunday. Further, any employer must obtain a permit from your local education authority (which means that your school must give permission):
http://www.direct.gov...entsRights/DG_4002945
Most big companies won't employ young people who've not yet reached school-leaving age because of concerns over such things as insurance. You'll probably need to ask around market stall-holders, newsagents, independent 'corner shops', hairdressers, etc.
However it's worth remembering that there are no legal restrictions upon you being self-employed. You could offer a car-washing, dog-walking or ironing service without any restrictions.
Chris