I used to use a detector and most of the ones I came across seemed to be off.
95% of my driving encounters only one camera so there's no need for a detector.
It's probably just the ones that catch a bazillion drivers a month making serious £££s that they bother with. I see the hadecs are up on the 50 zone part of the M1 near Northampton. They will catch alot once it's variable speed limit. A thin little yellow box that sits over the grass. Not even the lanes.
And no film so minimal manpower required once setup.