The simple answer is that the cop car will probably have a camera, and will be able to show what you did. If you are sure of your case legally then go to the court and argue, otherwise (or if you don't have the time or it is inconvenient) then pay it and walk away and avoid that situation from now on.
I have to tap that IMHO coasting down to 10 MPH, or anything under 40 MPH, on a "highway" seems a little too extreme - it is not something I do. I wouldn't coast to 10 or less except when coasting for a light and even then only when in town and with my foot covering the brake pedal - pedestrians always seem to want to get a "free ride" on my bonnet.
The problem is that normal people (i.e. morons) don't notice the speed of the vehicle they are catching until it is between them and their headlights - or it seems that way.
Honestly, I have cruised at 55 (PSL 70, normal traffic 60) and had cars only notice me at the last moment and swerve to the outside lane. These days if I intend to cruise slow or coast I find something big, slow and visible to follow so that people see it and move over.
Even with that I still get people only paying attention at the last moment.