I would suggest that you take a year out before going to uni to achieve the extra grades you need, possibly at a college.
You don't sound like you need to re-take too much so make the most of the year, you could even do an extra subject that interests you for any potential plan b) such as starting a language.
Take up a new team sport, or get active in another way which you can use at OASC to demonstrate your determination and commitment. Team activities come across very well on application. Perhaps even consider something like the Duke of Edinburgh Award. This will all help with the fitness too (which really isn't too tough given a few months of hard work).
Join the ATC if you aren't a member already. (I joined when I was 18 specifically because I regretted not doing it when I was younger and to gain a little insight into the RAF, not withstanding that I knew it would at least look like I put some effort in prior to OASC.)
Remember also that you can still apply to the RAF when you finish uni, or before, but with an offer to start with the RAF when you finish uni. Furthermore, plenty of people these days join the RAF without a degree but complete one with the Open University or similar during their military career (the military even puts a significant amount of money forward to help with this).