This may sound strange... but if a Uni has spaces for clearing, at say, a minimum of BCC (Birmingham for Biological Sciences for me) Does that mean that they would of accepted everyone who's done BCC or better in thier actual A2 results, who already have a conditional firm/insurance offer?
i don't work in the bham admissions dept. but this is what i think. based on experience and hearing other people's tales, it seems that they're quite willing to accept people onto a course if they drop one grade. it can be subject specific. if they wanted a B in biology or something, and that was one of the ones you got a C in, they might not be as interested.
in all honesty, i'd make a small bet that you'd be fine if you dropped a grade to BCC. i think they'd actually rather have people who have put the uni down as a firm/insurance and got BCC than someone who never applied/put it down and got BCC - it shows that you actively want to go there.