The police will almost certainly offer you a course for this offence (provided you have not done one in the last three years). This will cost you a few hours of your time and about £100. If that is not offered, or you do not fancy it, a fixed penalty of £100 and three points is the alternative.
If you want to challenge it, the only option is to decline those offers and plead not guilty in court.
The system which measured your speed is assumed to be working correctly unless it can be proved otherwise. That proof will have to come from you. You will have to show that it cannot be relied upon to measure your speed accurately enough so that the court is sure you committed the offence.
If you fail you will face a fine of half a week’s net income, a “victim surcharge” of 40% of that fine and costs which will be at least £620, and possibly considerably more if the police have to enlist an expert to counter your claims.
