Speaking as an A2 photography student I'd say including photography as a fifth AS is gonna totally mess up your workload. I know I'm just totally photography obsessed and abnormal in the amount of work I do in it (I've been adopted into the photography staff because of how much time I spend there
), but you need to be doing at least four additional hours per week per subject. Realistically, to get a decent grade in photography you need to be doing five or six hours (I average between 12-15 per week, but I'm aiming for an A* and I'm starting my degree in September). So you'll end up with 22 hours or so of independent study, which I wouldn't wish on anyone! So really consider if you can cope with that much work, and if you'll have the energy/motivation to continue doing that for a year.
In terms of course structure, I'm on OCR. So coursework runs from September to the end of January, Unit 1 is generally a project set by your teacher and depending on your course you'll learn either traditional, digital or a mixture of both before starting the main bit of the coursework. (I was the last year to do pure traditional at AS). Coursework is worth 60% of your final grade, and a decent size of that comes from critical studies and artist research - there's actually a lot of writing in photography, not all swanning about with a camera.
The exam unit runs from 1st February (that's when you're supposed to get the paper) and is the other 40% of your grade, like GCSE art you choose from a list of prompts/questions - some are more analytical or essay like, some are just single words. In May you'll have a five hour exam during which you print the final pieces and finish off sketchbook work etc, and then you're done. Photography tends to finish earlier than everything else, so you end up with a load of extra frees
You need original ideas, a new way of looking at things. Don't just regurgitate what other people do, look at their work and build on that, do something similar but let the work evolve. Try everything, don't throw any pictures out. Keep them all, but them in your sketchbook. Even if they're awful. You must do crap things before you can do good things.
I could probably answer most questions you might have, so feel free to ask