I think GCSEs are less important for Cambridge than they are at Oxford, as Cambridge like you to have high UMS in your AS grades. The going average appears to be around a UMS of 90+, but 80+ will sometimes suffice if you write a good PS and interview well – assuming your grades and PS were still good enough to get you the interview. If you can get some high grades in your AS subjects, then give Cambridge a go. I assume you're only just starting your AS levels, otherwise you'd have given more info on your attainment.