I think you might find that the what the radar detectors are finding quite a lot of the time is the instruments that measure the average speed of traffic in the variable speedlimit areas - they are pretty much on every gantry - which may or may not also be the type that take photos of you if you are speeding .
They do seem to be some of the more 'intelligent' cameras & work in a way that they catch out people who are going radically faster than the flow, rather than just people travelling fast & safely. Strange, but this does seem to be the case!

 

They are The gantry mounted gatso's on the M25 use the 'livelink' system which connects directly to the DVLA. They are not traditional film based units (let's face it, it would be a full time job replacing the film!). I think they provide a live feed, which identifies the number plate via the DVLA database. There have also been tests of non-intrusive (i.e. infra red) flashing systems, similar to those used on forward facing Truvelo's which means you cannot rely on not being flashed Not sure if this has been tried on the M25, but it wouldn't surprise me.

Incidently, the cameras in the roadworks around Heathrow racked up £600,000 worth of fines in less than 6 months.

Whether you agree with the use of the cameras in the variable speed limit areas, you would be very foolish to ignore them.

 

So to summarise:

From M4 to J10 (the original variable limit bit) there are digital cameras on some but not all gantries across all three/four lanes. It would appear they are not live when no variable limit in force but when lower limits apply they are. Radar detectors can see them when live. Origin B2 knows which gantries have cameras and which don't but can't know what variable limit is in force so will always tell you 40 limit. Geodesy just thinks all gantries have cameras.

From M4 to M40 there are gantries but no cameras?

From J10 round to I guess J8 there are gantries but no cameras.

 

Not a peep from the postman.
So unless anyone can say they've been caught I'm going to assume they're fake and ignore them from now on.

 

Never been done either. Usually don't spped roun there anyway (don't get the chance to )

Although last time I was in a rush to the airport at 3:00am I may have gone over the speed limit on a few.