What happens when you have a very minor accident and leave the scene without exchanging insurance info or calling police?

The passengers and drivers in the car in front are going to sue you for whiplash injuries (£1500 each) and get expenses for private appointments with medical specialists (£200-300 each), the owner of the car will manage to find some damage even though you didnt. He will at the very least claim for a new bumper even if it is an old car , he will get the money but wont replace the bumper so he can treat himself to something nice with the cash.(My bumper cost me £435 on my Volvo when I reversed into a low bollard).

If they involve the Police, you might be prosecuted for leaving the scene of an accident which resulted in an injury. Did you exchange Names, contact details and Insurance company details? I hope you did, you should have done.