According to my friend in the Metropolitan Police, if the camera didn't flash you, then the camera is not working. All rear facing yellow Gatso speed camera's, whether analogue or digital are required to flash by law if activated. Gatso's are operated by either piezoelectric strips embedded in the road; set a predetermined distance apart, which register the time it takes for your wheels to pass over them or by utilising a laser ranging system to measure your speed. Gatso's take a number of pictures, both normal and infra-red, the infra-red picture shows up your registration number regardless of how dirty your number plate is (Anti-speed camera number plates do not work), 2 normal pictures are also taken, usually half a second apart, which is why you see 2 flashes, these allow the police to calculate your speed and they do this by using the white markings painted on the road that are set at a predetermined distance apart. Using a simple formula and the 2 slightly differing pictures, the police can calculate your speed.
Only forward facing Truvelo cameras do not flash so as not to blind oncoming drivers, they also utilize piezoelectric strips embedded in the road and the first thing you know about tripping them is if a letter or fine drops through your letterbox, they also take away all doubt over who was driving at the time as there is less likelihood of contesting the prosecution as the photographic evidence identifies the driver as well as the vehicle.