I've changed hundreds of these over the years but this is first time I've seen one with it's backplate. Did you older guys used to throw them away back in the day ? :
Wylex 100A Double Pole Main Switch Isolator WS102
List of BS numbers for old isolators and protection Devices
Wylex Main Switch Disconnector 100 Amp Double Pole Isolator 100A 810
WYLEX 100A 50Hz DOUBLE POLE MAINSWITCH. CAT NO 810.BS 5419. L 1395