Defoo DVLA checking your tax, their cameras are on tripods, usually small and round in shape and behind the vehicle which is parked up. This is the usual one parked up on the A48M in Cardiff:
A speed camera will consist of two units. The first will be a laser device usually rectangular in shape with 2 lenses. Next to it will be a camera with a nice lens attached to it.
White cameras on tripods on motorway bridges are ANPR cameras, these do not check your speed but they do check tax, insurance, MOT, and any markers on the car. Lots of police cars and motorbikes will be hiding further ahead to stop you.
I wouldnt worry at all....
One word of warning though - I did once on the M6 North see Police using Speed cameras on tripods on a bridge. They were then radioing the cars up ahead to stop speeding drivers. I was doing 90 at the time and slammed on the anchors at it was just after 7am, thankfully, all six, yes, six police cars were in the hard shoulder with their victims handing out tickets. I think they were relaying the picture from the mobile camera to the TV unit in the police car as evidence.
Generally by rule if its any other sort of speed camera, the police have 14 days to get the NIP (notice of intended prosecution) to you, if it arrives a day late, they cant do you. If you do get one, check EVERYTHING on it, even if the road name, junction location, anything on it is incorrect it can be thrown out of court.