I think you will find that the only hope you have of winning this case is if the speedo was incorrectly calibrated in the cops car and good luck with that.
Other than that a cop following you for a set time and matching speeds with your car has always been considered as a valid way of estimating someones speed in a court of law. In fact before the advent of radars this was pretty much the only way of them correctly telling what speed you were going.
If you plead not guilty and lose, you will be fined the maximum penalty plus court costs!
I'm afraid your going to have to bend over and lube up for this one as you really have a snowballs chance in hell of winning sorry to say.
But you can blame all the tossers in imports who think that it is cool to do skids in residential areas, do the traffic light grand prix, go drifting around roundabouts in the rain, who make their unfeasibly loud blowoff valve go off for no reason other than to draw attention, race on the M1 ..... etc etc