I couldn't help but notice when I first went to the UK the lack of Police cars on the highways. The law seems to depend on cameras! That would make more sense if there were no warning signs!
In my part of the world, the cops have unmarked cars of many different makes and colors so if you are speeding, you never know who is behind you.

One woman from NJ was getting a ticket for sppeding on the Maine Turnpike, and she said to the cop, "Don't you people ever give warnings."? He replied "Yes, we do," Pointing to the speed limit sign, he went on, "There's one now."