Camera not in use sign with speed camera flashing cars

Last night about 9:30pm I and several others were flashed by a speed camera on the M4 between Bracknell turn off and Reading Services. I can't be more accurate as I do not know the area. Signs were showing 60 mph and my limiter was set to this and I was flashed by a speed camera along with many others traveling along at the same speed. It was very dangerous as the night sky in my mirror resembled lightening but the worst thing was the sign by the speed camera which stated "Camera not in use". Very annoying to be caught at the correct speed limit and with incorrect signage by the camera. I was angry last night, evenmore so today. If a 60 limit sign is there and my car says 60 I expect it to be correct with 10% error so 57 to 63mph. I do not expect a flash until I am doing 64+. However I do not expect to be flashed and prosecuted for doing the correct speed. Paul Smith