Sam,
When reversing during the driving test, if you notice that you are going to be a lot further away from the kerb, then it is alright to stop reversing and go forward a little to correct your steering. Examiners do not expect perfect drivers but as long as you can rectify your error(s), then you will certainly pass.
Depending on the driving examiner, it can be marked down as a minor driving fault or may be ignored completely if you're a bit far from the kerb. However, in certain situations, it can be marked down as a
major fault if an accident could have been caused by you being a lot further away from the kerb. An example of this could be that you have reversed in a road and you are so far from the kerb that your vehicle is literally on the opposite lane (that is on the right) and an incoming car had to brake sharply to avoid crashing into your car.