That's interesting, one of the classic images for many of us in the states is that you always ask an officer to see his badge and you get his badge number. I suppose detectives don't wear their badges on their shirt, but they carry it in a wallet. So from that point of view you could say that a detective (say on the Law & Order TV show) is "unmarked" because he carries his badge in a wallet and not necessary right on the outside of his jacket.

yes, that is the idea of plain clothed police, as we call them here, and i can see that they are absolutely necessary. the guys i'm talking about are riot officers who come out when it gets messy. there is no excuse for making them anonymous.

That is very true. If someone is charging at you in riot gear it is fairly obvious that they are with the police (one would hope). And yes definitely if you had a serious problem with one of them, you would want to be able to say which one in particular had caused the issue for you.

I found a UK newspaper making that point - that all riot police should have large ID numbers on their front and back -

http://www.dailystar.co.uk/news/view/77520/Make-riot-cops-wear-footie-style-numbers

I agree with this.