Find out if you can appeal without it going to court.
If not do the sensible thing and pay the fine but make a complaint.
Explain you only paid the fine to avoid court but do not accept you are in the wrong.
If you are innocent state your case as its easy nowadays with digital pictures and email. Bear in mind that others may have complained about this policeman and your complaint could be the one that makes them investigate him.
Stand up for your principals but do it in a relaxed way using the tools at your disposal, that way it wont wind you up and think of the satisfaction when you prove the point that they could not see from behind.
You dont need to pay for legal help just present your case yourself with your evidence stating what the guy said.
If you get no joy then involve your MP. When you write to him he has to answer and you can explain that you are approaching it this way as legal costs to protest your innocence are too prohibitive.
I`ve written to my MP twice and they have to see it through. Thats what they get paid for.
Yes its hassle but take your time and make your point. Trouble with the world today is not many people can be bothered to make a stand over a principal. Just keep it in perspective and dont get wound up.
If nothing else it will act as therapy for the anger you feel at the moment for being accused of lying. If you didnt, prove it..... It will be worth it