They are on for sure mate, not only when the limit is flashing, but all the time. They must be set with a tolerance, but ive seen plenty of people caught, especially early morning when the roads clear.
Another place ive seen with fixed average cameras is the elevated 50mph section of the m4 by Port Talbot, there used to be gatsos, but they have been replaced, and the cameras are mounted on the gantry bridges on both sides.