(2-part post):
Let's start with the rules:
Anyone who employs you must notify the local eductaion authority of their intention to do so. If the LEA are happy with the arrangement, they'll issue you with a permit. They can withdraw the permit (and stop you working for that employer) if your school work seems to be suffering or if they're unhappy with some aspect of the work (such as safety provision).
You can't work before 7.00am or after 7.00pm.
You can't work for more than two hours on any school day or on a Sunday. On Saturdays (and on weekdays during school holidays), you can't work more than 5 hours. (When you're 15, that goes up to 8 hours).
At some time during the year, you must have a continuous period of a fortnight, during the school holidays, when you don't work.
Those are just the main rules. The full details are here:
http://www.direct.gov.uk/Parents/FamilyIssuesA ndTheLaw/ChildrensRights/ChildrensRightsArticl es/fs/en?CONTENT_ID=4002945&chk=AaZA0p
Back to your questions:
If an employer intends to take you on (other than illegally), you won't be able to 'up your age' because the LEA will have a record of your date of birth and will take this into account when considering whether to issue a permit.