Theoretically you could be charged with 'fraud by false representation' (which is a serious criminal offence). In practice though, it would be the employer who got into trouble, and not you.
At 14 years old you can't work without a permit issued by your local education authority. It's the employer's responsibility to ensure that such a permit is issued.
For the rules regarding the employment of teenagers, prior to their school-leaving date, see here:
http://www.direct.gov.uk/en/Parents/ParentsRig hts/DG_4002945
Jobs for teenagers are rarely advertised. The usual places to seek them out are newsagents, hairdressers, caf�s, independent 'corner shops' and market stalls.
Some young people make money through self-employment (e.g. by offering a car washing service to local residents).
Chris