If Eeyore had straight Cs, then yeah, it would probably not be worth her time applying. However I know what GCSE grades she got and they're pretty good. Better, in fact, than hk_ghetto (no offence to you m'dear, hope you don't mind me using you as an example) who has, if I remember correctly, just got an offer from Trinity Cambridge. No mean feat.
I wish people would stop putting potential applicants down like this. Bono and Tek, neither of you have been through the process. Even people who have couldn't possibly presume to speak for an admissions tutor! Just try to be a bit more supportive, eh?
But yeah, Eeyore, the interviews are what count so prepare as best you can for them by having as thorough a knowledge of your subject as you can