I'm in year 12 and am currently studying for 5 AS levels. I got 9 A*s, 3 As and a B at GCSE and am told by all of my teachers that I am 'of Oxbridge calibre'. I too thrive on being intellectually stimulated. However, when I needed money, I went for whatever I could get. I worked in a shoe shop (Shoe Studio at Debenhams no less!) for a measly £4 an hour. Some customers were downright cantankerous and treated me as if I was an insanely unintelligent person with no GCSEs, just because I was in charge of a shoe department on Sundays. I don't even come off as unintelligent, they just presumed that immediately. Nice.
Anyway, I'm agreeing with everyone else on this thread: James (sorry to get so personal :P), everyone has to start at the bottom. Try and make the most of it and enjoy it if you can (I found my solace in amazing co-workers with my astounding abillity to take off any percentage in a few seconds; Debs likes its '35% off' indeed, and in C1 you either get good mental maths skills or perish, but I digress...). You won't get the stimulation you desire, but you'll have to put up with that and try to prsoper however you can. Good luck!