I got caught doing 86 in a 70 and got £62 fine and 3 points if it's any help... (this was 6 years ago)

 

If more people (read everyone) contested these things, the whole system would collapse. If you're in any doubt its worth the risk of a bigger fine and more points to receive the judicial process that you are entitled to.

It would be very bad luck to be caught accidentally at what is going to be indicated around 100. Make them PROVE their case and good luck.