Just to add to what Spie has already pointed out, the Cameras on the Motorways are often just up on an overhead gantree, the only notice being standard speed camera warnings. This is definitely the case on the section of the M6 Junctions 5-6. They must catch thousands a day

Some peps, not knowing think that because they know where the cameras are, they are easy to spot as they've now included a yellow square background to them, that they are safe if they go slow past them. NOT SO! On this section there are at least 4 cameras in each direction and your speed is averaged out through the section by using timings from all of them.
The posts with the upside down W were used on the M6 some time ago up a junction 19 for a temporary installation.
As Spie said the only way to detect them is with
Geodysey or
Origin Blue
Good luck and watch out for the dreaded SPECS.
Loz