Well you could decline the fixed penalty offer.
The police will then prosecute you in court and the FPN will play no part in the proceedings.
The guideline penalty in court is half a week’s net income. You will also pay a surcharge of 40% of the fine and costs, which will be around £95 if you plead guilty or around £650 if you are convicted following a trial.
£100 looks to be a bit of a bargain!