So was heading to a wedding on Saturday, sitting on an A road with police traffic car behind me. Coming the other way round a corner is a car that seems to be enthusiastically making progress. He seemed to brake, I assume when he saw what was behind me.
So me and Mrs 100th wondered if he could be done for speeding. Would the traffic car be able to? Mrs100th is a member of the thin blue line and reckons if they were going to do him for speeding then they’d have gone all woop woop and given chase.

Don’t know of any onboard tech that could do them for speeding. Vascar needs a set distance in persuit and the static cams need the vehicle to be static. Video evidence of them being a lunatic could be used for careless / due care. If it was a traffic car and if they’d not slowed down then I’d have expected them to head after them. Normal patrol car would have no hope. The fact that they did slow down is half the purpose fulfilled.

a moving police vehicle ned 2 people in the car one to drive and one to observe, also the speed measuring device needs to be on, and the car needs to be followed for a set distance.

stationary speed camera vans dont need this as speeds are recorded and filmed.

Ah well seems I was wrong. Mrs reckons it was near end of shift and they’d have had to pursue in the wrong direction.

Fairly common reaction. Like the bubble of 70mph you get on the motorway around any marked up car.

A Vascar unit works over a measured distance divided by time. Divide one into the other to get speed. In the OP’s scenario a patrol car will not have been able to measure the distance so thus is unable to calculate speed.
A British Traffic Car is also unable to use either Lazer or Radar devices whilst it itself is moving. (Unlike a Highway Patrol Officer in the States).
The only other way of the opposing car being reported for speeding without the use of a Vascar or the like would be on the ‘expert opinion’ of the patrol officers.
However this is very difficult to prove. Any good solicitor would question the Police on their level of competence relating to being an ‘expert’.
Technically in any prosecution for speeding, the case is based originally on the ‘expert opinion’ of the officer, this is then corroborated by an electronic recording device wether it be Vascar or Lazer etc..