When i was at college (Which was a good number of years ago

), the lecturer we had said the speed limit had to be at least +10% because speedos aren't calibrated once they leave the factory. So any conviction for speeding at anything less would be quite easy to challenge as the driver would just need to say i was doing what my speedo said, and there is no legal requirement to get any calibration done on it.
But before anyone goes 77mph in the hope you are 10% under then you may find you are actually over and thus doing a lot more than you think. Saying all that, how would you stand if you could then prove your speedo was showing 30mph under for example. They can't really say you was speeding if you was doing what the speedo said was the right speed.