There are various guidelines about where speed cameras can be located but these principally relate to static cameras. The only rule about the location of speed cameras is that, if a local authority wishes to retain the monies received through fines, a static camera should be prominently sited and painted yellow. Even so, if it was found that a static camera didn't meet these criteria, it wouldn't invalidate any speeding ticket from that camera.
Here in Suffolk, the police park unmarked vans in laybys, with speed cameras facing out of the rear windows. It certainly doesn't break any rules and I don't think that there any guidelines which seek to prohibit this.
Chris