Does anyone know whether the mobile police speed camera vans can take pictures through the dark windows at the rear or do the doors (rear/side) need to be open or the camera outside?
Go here:
http://www.ukgatsos.com/
then click on "Links", then "How speed trap radar works". Maybe that will give you the information you are looking for.
Most vans either have the doors open or 'letterbox' flaps in the back windows, but beware! It is not unknown for you to be approaching the rear of a van, doing 28mph(!), go past the van and speed away. At this point the operator spins the whole camera rig and gets you disappearing into the distance through his windscreen! Result: Summons, �60, Thank you very much!!