Sorry to say but unless a police officer witnesses something, or is directly involved in the incident, no prosecution will be bought. Even with video evidence or photographs, they will not be deemed credible evidence as lawyers could argue they could be fabricated or not 100% genuine. Even if they do not, the "Police" tend to be the only credible witnesses as they are deeemed to be "Trained Observers". Don't shoot the messenger, because even if I make a call, off duty to report that I am following a vehicle concerned in Dangerous Driving, I will be told (and have been) "The vehicle will be circulated, thanks for your call".
Unless the driver has committed a CRIMINAL OFFENCE and not a TRAFFIC OFFENCE, it is unlikely the police will take action.