@questioner, there is generally one resit allowed (not sure if it's mandated but that has as far as I known always been the standard).
However there are never gauranteed second chances, there are cases where training is suspended and the trainee is recoursed onto another course or into another disciplin. However there are also many cases where training is not resumed and the trainee terminated.
The harsh reality of it all is that if you make it to a basic course start you are by no means gauranteed to make it to unit or to validate at that unit. All I can say though is if you make it that far don't fail on theory. There is no real excuse (in my mind) for failing the theory, most of it is fairly simple it's just a case of memorising large chunks of data. Anyone can be taught ATC theory, the practical side is what gets people and if you fail on theory you've not even given yourself a proper chance at the job.