With speed cameras, I think it is very hard to dispute unless you have something showing proof, such as your registered GPS and get the history. I am thinking that you use the cruise control, what I can suggest (long shot) call the car company and ask if you can download the history of the cruise control. I am not sure if there is such a thing, but this came to my mind, as most vehicle now have all the new tech.....
Other idea, also long shot, do you use dash camera? if so and you have saved the video, you can calculate your speed and use that to prove you were not speeding.....
let us know....
hope this helps.