For cambridge, it depends on your GCSE grades in context to your schools average, your circumstances etc. Cambridge is not the be all and end all though. One way you could potentially make up for GCSE grades is getting a really good LNAT score, as the reason they look at GCSEs is due to it being the most recent offical examinations you did, but the LNAT is also an offical examination that lets them judge how you are doing now, as predicted grades are typically over inflated and they know this. Good luck with it all!