The Application
I'm following this up with tips on the UCAS application partly because it's been a couple of weeks, and partly to procrastinate from revision. It would have made more sense for this precede personal statement stuff, but imo it's far less important than getting started with reading (I started over the Xmas holidays a year before the interview, and it took a LOT of time to learn how/ what to read, lol -- giving yourself space to try out different things is crucial).
1. GCSEs: in truth, they're really not that important (provided your predicted A2s are strong), especially for this year's applicants who didn't sit them. Oxford does place a little more weight on them, so Cambridge might be a better idea if only if your GCSEs are well off "Oxbridge-standard", but even the ELAT is more important and you can't change them now. In short, don't stress about GCSE results! It's far more productive to concentrate your efforts on reading, ELAT prep, interview prep etc.
2. Predicted A2s: you'll need to meet the entry criteria (AAA for Ox A*AA for Cam). Beyond this, predicted grades don't count for much. Remember that everyone else applying will almost certainly have A*AA-A*A*A* (probably the upper end): the A*A*A* kids who perform worse than A*AA at interview will be the rejects.
3. SAQ (for Cam only): Largely an admin tick-box exercise. Just remember that you'll have to put all your A-Level texts in, so don't try to pass off one of these in your PS/ interview as wider reading. (On a side note, make sure the photo you'll have to put on it is one you'd be fine with on your ID card (which it will be if you make it to cam). I didn't really think I'd get an offer, so I was fine with my sister taking a cr*p one of me in my sweaty sports kit when I got home...)
Everything else is probably intuitive. What I want to stress is that you only need
1) A 4 (C-grade) in GCSE Maths and Eng. Lang.
2) Compatible predicted grades
3) A willingness to read well beyond the A-Level curriculum to an undergraduate level.
Anything else will help your application, but isn't necessary to gain a place.