BS546 is very uncommon now, but can still be found in some relatively modern british homes and businesses where the sockets are used to "code" for connected appliances. For example, the 5A socket may be wired up to a switched lighting circuit to connect lamps but prevent connecting higher power appliances. I've also seen the 15A sockets being placed in communal areas of flats to provide cleaning and maintenance staff power while discouraging tenants from using them.