How do you plan to fight it?
At this stage, all you can do is respond naming the driver. You should then be offered a speed awareness course (c£90) or a fixed penalty (£100and 3 points).
The only way to fight it is to ignore or reject those offers, and plead not guilty when it eventually reaches the court stage.
The police will undoubtably produce photos showing you passing the gantry at the alleged speed. How will you cast reasonable doubt on that evidence?
Bear in mind that the cost of the likely failure will be nearer £1,000 than £100. Plus a day of your time, win or lose.