These plugs and sockets are based on BS1363, but have at least one pin of a different size or shape making them physically incompatible.
Click Scolmore non standard plug and socket
Click brand 13A plug with non standard earth pin, made by Scolmore.
Britmac non standard plug
Britmac branded 13A plug with round line and 45 degree angled neutral pins.
MK non standard 13A plug and socket
MK non standard 13A plug based on BS1363 with T shape earth pin.
Round earth pin non-standard 13A plug
13A plug and socket with a non-standard round earth pin.
Walsall Gauge 13A plug and socket
Walsall Gauge 13A plug and socket with all pins rotated by 90 degrees.
Wandsworth 13A plug with square earth pin
Wandsworth 13A fused plug with non standard square earth pin.
These plugs were typically used in locations where it would be undesirable to allow people to connect their own equipment. This is either to prevent people from using the electricity supply, because the voltage at the socket is different, or to prevent interference with other equipment on that circuit.
Common or shared areas of apartment buildings was a common use for these - sockets were provided for cleaning equipment and the electricity paid for by the owner of the building. Non standard outlets prevented those people living there using the sockets for their own purposes, such as connecting a long extention lead and using the building owners electricity rather than paying for their own.
Those with dual fuses such as the Walsall type have been used on circuits with voltages other than 240V. One example being on the London Underground which had 110V, arranged as two 55V supplies referenced to earth in a similar way to UK building site supplies.
Still used today, non standard outlets are used for some computer and medical equipment. This prevents items such as vacuum cleaners creating interference on the circuit, and in others avoids overloads such as where a circuit may have backup power in the form of a UPS or generator system.
This video shows the various types of non-standard plugs and sockets, with the exception of the Britmac style.