Don't panic too much - if your A*s are in science at GCSE and that's what your doing at A-level, then you are competent enough to get AAA.
If you have failed, retake. You may have to rethink your options for your applications as some don't allow you to apply with retakes (I retook 2 A2 modules from January and have an unconditional offer, so it can be done).
But at the same time, if you got below an E in chemistry, you have to think whether you have what it takes to be able to get an A by the end of next year. Was there a reason you did so badly? Did you not revise enough?
At the end of the day, see what you get, and then talk to your teachers about their expectations for you over the next year. If you did as badly as you think, they may not predict you high enough grades to get an interview.
If that is the case, again, DO NOT WORRY.
If you then get the grades, take a gap year and apply again. Not only will it allow you to save money for uni and become more mature, but it shows how dedicated you are to becoming a doctor, which is really important!
I've gone through the whole thing twice (went to uni in my gap year as well), so if you need any advice or anything at all, just message me