Im pretty sure you do not need 6A*s to study medicine, but you will have to be quite selective when picking where to apply. For example, Cardiff look just at GCSEs, and the amount of A*s needed to get an interview is about 6 (i think), so you shouldnt apply there. Make sure to look at academic selection criteria on the uni websites.
Just try to do as well as you can in AS's as that will help and mean there will be unis where you are a very compatible applicant. If not, you can get a very good personal statement as well as UKCAT or BMAT score