It is important for the police to obtain as much information as possible with regards to the details of the car/bike and also the driver/rider and passengers. You do not have to disclose your details to the police if you are fearful of reprisals.
It is best to contact the police as soon as the cars/bikes arrive so that the police have a better chance of apprehending those responsible.You can report nuisance motor vehicles to your local police force via their 101 non-emergency number.