Whether or not to call the police seems to be a grey area for most people when they are involved in a road traffic accident. It is always advisable to make a report to the police even if it is a minor accident where no one is hurt. However, it is very important to call the police if someone if the other driver leaves the scene of the accident, if you think the other driver is under the influence or has no insurance or you believe the accident may have been caused deliberately.
In any event, the police should be informed within 24 hours of the accident. Failure to report an accident is an offence so care should be taken in ensuring that this protocol is followed.
If the accident caused damage or injury you are legally obliged to share your name and address with everyone involved in the accident and swap insurance information with the other driver. You should also try to find out if the driver of the other vehicle is the registered owner of the vehicle. If they are not, you should try to find out who is the registered owner (it could be a company car for example).