I was speaking to a copper about this recently and he said in the eyes of the law there is no such thing as an accident. It is a road traffic collision and all of them are meant to be reported to the police and they will then take a view as to whether any further action needs to be taken. He said its the law as it stands.
Obviously they don`t tend to get involved in something that is just a small coming together but if its more serious then they take a view on whether action should be taken to stop it happening again, such as an awareness course. Or they may just prosecute because they feel the outcome requires it and you need to suffer the consequences.
Hopefully he was wrong and you are not obliged to report it. I know i would never involve them unless forced to as they always seem to make matters worse
I can guarantee you that not all accidents are reportable to police, only if there is injury or if insurance details are not swapped are Police needed to be contacted.
This comes from being a cop and unfortunately having far too many calls about minor RTC's.