If it's a standard GATSO camera, then I think it works like this:

Under the road on the approach is a pair of detector wires, which detect the inductance change caused by a big hunk of metal (your car) passing over them. By comparing the time between each wire detecting your car, it can tell how fast you're going.

This causes the camera to take two pictures at a fixed interval. The guy who develops the film (probably not Boots or Jessops) notes down your number plate from the photos and generates the ticket.

Now, at this point, you're already speeding, and you're gonna get a ticket (assuming that there's a camera in the box, and film left in the camera).

The precisely spaced lines painted on the road, and the precisely timed flashes of the camera are used to work out exactly how fast you were going, in case its borderline, or you contest it in court.

So, essentially, assuming that this was an old GATSO, you'd already have triggered the double-flash by driving over the detector, and the only thing that slowing down did was muddy the waters a bit. If you came to a complete stop, while still on the lines, you probably spoofed it.

Now, odds are that the box (even if it did flash) didn't have a camera or film in it, and you've got away with it.

The newer cameras (the ones that face you) are calibrated accurately enough that, if they take a picture, you were speeding, and it's game over.