All vehicles registered after 1st September 2001 need to have plates that are physically made of 'retroreflective' material. No metal is 'retroreflective'. So I'm guessing your plates are on vehicles registered before that date then?

In any case, this cop made what I would call a bad decision - he was perfectly within the law but I can assure you that if he went back to the station and told 10 other cops what he'd just done (with all the attendant circumstances), at least 9 would think he was a prat and 1 or 2 of them would tell him (I'd be in the 1 or 2 )