A lot of mark scaling goes on. I don't know what the effort difference between getting an A or A* in geography is, but last year when I did AQA spec A geography, I only did some last minute scanning through my textbook just before the exam (apart from that, I didn't do any revision). I went in to the exam, felt like I didn't even know the answer to a single question. Guessed some answers, improvised some, and still got an A. I even did stupid things like writing a single line answer for one of the 7 mark questions (I think its 7 marks, is it?).
I ended up with an A. I did get 25 out of 25 in my coursework though. So I think you could get an A* if you revise like crazy for the exam. Otherwise it is an A, which is still excellent (an A* is just the upper part of an A!).