http://news.bbc.co.uk/2/hi/in_depth/8000246.stm "... all uniformed officers must wear shoulder identification so they can be easily identifiable by the public."
This is obviously taking one from American football. (they don't wear big shoulder numbers in either UK football or cricket.) The idea has a lot of merit tho, here as well as there. Even if it is event based (you get a new number every day depending on assignments) being able to easily tell which cop is which would go a long long way to reduce brutality events, and remind a cop he isn't a omniscient being, but serving the public good. Here, the cops are all worried about protecting their true identity from the bad guys. This would allow masking true identity without obscuring liability.
It's a good enough idea, I think I'm going to write someone about it.