Why do we call them coppers?

I think that the the slang for the police ‘copper’ comes from cockney rhyming slang. Coppers = Grasshoppers, and hence the terms of ‘copshop’ or ‘grass shop’, directly deriving from ‘Bottles and Stoppers’ (coppers) and then the term ‘grass’ deriving from the slang for someone who grasses someone else up etc.