As New Judge states, criminal law, relating to motoring matters, applies on a petrol station forecourt because it's regarded as a 'public place' for the purposes of the law. So, in the unlikely event of the police becoming involved, you could be prosecuted for 'driving without due care and attention'
Civil law applies anywhere. If another person suffers loss or damage through your negligence, they have the right to demand compensation. Quite clearly, you should have checked your mirrors more carefully before reversing. (Even if the other car wasn't parked right behind yours, a pedestrian might still have crossed behind your car).
You must ensure that the other driver receives compensation for the damage to their car, either by paying for it yourself or by making a claim on your insurance (which is just as valid on a supermarket car park as it is on the public highway).
Chris