This is an interesting article that may give you more information.
http://www.speedcam.co.uk/gatso6.htm
Im pretty sure that I read somewhere that if you swap lanes between the camera,s they cant do you. As they measure the speed in a straight line between the two sets of cameras, if you swap lanes then your distance travelled is greater and the law has to have an exact speed for the exact distance travelled, but dont take this as fact.
These cameras are supposed to make the road safer but on motorways they are in my opinion dangerous. Too many drivers are forcing their way back in to the inner lanes before the camera,s and too many brake which will sooner or later cause a pile-up