Metal switches should be earthed.
You should not use the screws as the only earth.
With a plastic back box, the screws aren't connected by the back box anyhow.
I've never wired in a metal switch, and I can't find a photo online, but I can't see how they could possibly meet the rules without having an earth connection.
House wiring has had earth wires to switches and lights for the last 40 - 50 years, so it's almost certainly just a matter of joining the earth wire that's already there to the earth connection on the switch.
If either is missing, get a man in.