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rhowe
Nef
You don't.
For a start, the original fitting has 3 live termials - Loop, Live and neutral, and an earthing point, but your new accessory only has terminals for live, neutral and earth (the loop is used so that you can take a cable with mains power to the light without powering the light itself, and then take that power to a switch and back, and then to the light itself).
As has been pointed out, the old wiring is 2 core cable that is just live and neutral without an earth (CPC) so it would be dangerous to use a metal fitting as it may became live in fault condition (live or neutral conductor touching the case). I've literally just spent the last week updating a property so that their lighting wiring has an earth, as they wanted to use metal faceplate light switches and metal light fittings.
Pictured below is an example of the bodge that we found. There is no earth at the switch, but for some reason they've put an earth link between back box and faceplate - which is worth fuck all if there isn't an actual earth connection there.