If you are referring to the camera installations mounted as a pair of boxes, usually painted yellow, on an arm pointing away from the motorway, I rather think those are HADECS cameras, and they are not average, but spot, speed detectors.
There are numerous such installations on the M1 and M6, and a high percentage of them do not have associated road markings. Does anyone know whether that means they are inactive, or do they not need road markings to detect speed?
While I'm on, the gantry-mounted average speed cameras in boxes behind the speed limit signs on at least the section of M25 between junctions 16 and 10 seem to have been removed, or at least I have not seen one flash for a couple of years now. There are now HADECS installations on this section, and I have seen several flash in that time.