A traditional Gatso camera relies upon taking two photographs (and being able to check the vehicle's speed, if required, by its progress over the road markings), which is why there's a double flash.
Such cameras can't catch vehicles moving towards them, as the double-flash in the drivers' eyes could be dangerous. (Even a vehicle accidentally caught, as in your post, wouldn't be prosecuted for speeding - but the photograph might be used as evidence of excessive speed if the vehicle was involved in a serious accident just along the road).
However the newer forward-facing Truvelo cameras use a different technology. If it's one of these
http://www.speedcam.co.uk/g43.jpg
and you're speeding, expect a letter!
Chris