Depends how bad; if it was just a hit and no damage to the car then i suppose the guy wont care if he even notices.
However if you damaged the other car, then you should probably phone the shop or the centre of the car park and ask them too see what the best method is. Or as someone suggested call 111 but phone the shop first.
For future reference; if you hit a car just leave a note on the windscreen and note their registration. Had you done this you could have phoned them and sorted it without 3rd parties. Although insurance companies state you MUST tell them if you have a accident this way saves you the hassle, however if the other person phones his insurance you have ****ed it.
EDIT; I doubt you will get banned from driving, and you dont have any no-claims to lose, but your premium for next year will be very high. Also dont stress too much over it.