As someone who went from BCD at AS to A*AA at A level, this is totally do able. I would suggest using your frees wisely now without too much extra curricular studying so you don't get burnt out, just use them to make up the knowledge you obvisouly didn't have in year 12. Then about a month before your first exam you're going to want to absolutely kiss goodbye to your social life and do around 6 hours of revision a day spaced out, moving up to 9/10 in the exam season. I was the exact same as you, never achieved an A in any exam I took however I always knew I had the potential, but just never applied myself.
Message me if you want to speak further.