In a Cambridge applicants thread, someone had posted a table of grades acquired at GCSEs and A-levels from successful offer holders, and some people have gotten in with 0A*s in GCSEs.
I'm not knowledgeable on this in anyway compared to Reality Check and Doonesbury, but I'm pretty sure if they look at your GCSEs which are mediocre, then at your A-level predictions which are great, they see you made a massive improvement and you have potential to succeed.