Where can a student get a job in London

[lawstudent]

hi, im doing my A-Levels now and getting all A's. i want a saturday job to get a lil money behind me. but all job ads in places i look ask for experience which i do not have, it gets so annoying as a would be such a good employee. all they look at is age n dat bt im so hardworking and full of backbone but they cant see that so i dont stand out from any other student where in reality i do. is there any big companies that take in young students or any agencies that can definatly get me a job?