Currently the M4 is being 'downgraded' to a smart motorway.
The limit is 50 and I've found that when I set the cruise to be exactly 50 according to my Garmin and phone GPS my digital speedo is registering 53.
I drive from J3 to J10, and return, at GPS 50 all the way and if anyone behind tries to push me they can talk to the car as It maintains a steady speed.
What I do see from time to time is the ones who come screaming up to a Specs camera, hit the brakes for the camera, then zoom off again. What they don't realise ..........