Thanks for all the helpful replies.
The 'copper' blanking plate is in fact brass but with a copper colour finish. There is a clear plastic gasket with it. "why copper?". Well, we're going for copper recessed lights, copper plug sockets and switches, copper kettle etc as a colour scheme to go with the grey/neutral kitchen.
Yet the plugs to go into those sockets will be black or white plastic! Makes it all rather pointless.