as what bully said... cope the fine and dont do it again!...
if you are smart man do above...
if you are stoopid challenge what police said, and proove they are in wrong.
but it is not worth the hassel. you want to go throught for small thing like not displaying plate...(sorry it polly big thing for you) as I work in law firm I see things worst than simple traffic fines.
but Good luck mate!
Just ask yourself that; IS IT WORTH IT!
cheers
Joe