Hmmm, yes I always had heard that they were illegal to use, but thinking about Im beginning to wonder...
AFAIK the legal requirement is for the number plate to be visible by the naked eye from a certain distance at street level, and for the lettering to conform to spacing and typeface regulations.
I cant see how that affects the fabled anti-flash plates (if working ones even do exist). If you can read them normally, its just too bad if the police cant photograph them, surely?
Perhaps a case of the CPS trying it on and securing prosecutions with no justifications in law? I know cos they tried it on with me, the secret is - dont ever buckle under the pressure.