any road other than a residential road or specifically designated speed limit (marked by speed limit signs) are 60Mph except dual carriageway or motorway which are 70MPH.
To answer your question, Interior cameras on police cars measure police car speeds like a speedo and not other cars speeds, Police use them on video camera to be used as visual evidance to show how fast they were travelling in comparison to the car they were following (i.e doing 80MPH in a 30MPH and the car in front is maintaining its distance therefore has equal or greater speed to the police car)
The only other option they have is to use a device which they can press on the camera that when a car they are following passes a specific point (such as a speed detector marking..The Diamonds/Squares u often see on dual carriageways) they press it and when they hit the second set of markings they press again gives them an average time and distance thus allowing the camera system to give an average speed. Again this is following a car from behind.
The only other camera is an external camera to detect speed and these are nearl always located at the rear corner of the police car and pointing rearwards. It is unclear but thought unlikely these will work effectivly without the police vehicle stationary first.
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