Check your paperwork and what your obligations are, but if you can, I would wait as long as possible before informing the RAF. Don't assume the worst, but follow the recuperation programme to the letter and see how the ankle performs under fully fit pressure.
As I think you know, any joint problem is a pretty certain no for Regt - but not a certain no, which is why I advise getting yourself in the best possible shape so that you can tell OASC what you can do, not assume what you can't do.
You should also consider if you want to go for any other branch. It is possible that you can get yourself into a fitness situation where you could be fit for other branches, except Regt. If you had a strong OASC pass, do you want to talk them into letting you apply for another branch?