It depends on which of the nine types of speed camera currently in use in the UK is emlpoyed to catch you i.e. Gatso, Truvelo, SPECS, Peek, Speedcurb, Watchman, Traffic Light, DS2 and Mobile.
There are three main methods that UK speed cameras use to detect a speeding motorist;
The first method used by Gatso and mobile cameras is to send a Laser or Radar beam at the passing vehicle. The beam is then returned back to the speed camera equipment, providing an exact speed.
The second method used by Truvelo and DS2 is too use loops in the road, if the passing vehicle drives too fast over the loops, the speed camera is triggered.
The third method used by SPECS is to take a photograph of all passing vehicles at point 'a', then several hundred metres along the road at point 'b' a second photograph is taken. Both photographs are date and time stamped, the speed camera equipment then calculates your average speed.
http://www.speedcamerasuk.com/speed-camera-typ es.htm