Like anything else legal mate, don't worry about it unless you get the metaphorical knock on the door...
Anyway, these cameras wouldn't be much cop if they always took pictures of the car behind the one that was speeding... Seem to recall Top Gear tried to work out how fast it would need to be to get through without being caught and it turned out you needed to be doing 175mph! Based on that, any picture the camera has taken should be of the speeding vehicle - the ambulance. Both its pics would have been taken before you entered the shot - or the courts are in for a hell of a time...
cheers
Dave
By the way - I'm no lawyer, this is all personal opinion...