I think my brain just exploded as I tried to read that.
THis is how it is. The coppers are well within their rights to estimate your speed and serve you an infringement notice based on what they believe.
Unless you have any evidence to the contrary, ie. a GPS readout or video or Allah giving testimony on your behalf, you're done. Or you could take a chance with one of the traffic infringement specialists and hope they can find a technicality to get you off. I wouldn't bet my house on that happening.
You could go to court and admit to doing 80. Again, it's your word against the police so you probably won't win, but you might luck in with a magistrate who got a root the night before and is feeling chipper.