British BC plugs don't conform to any British Standard, which is why they vanished from the market place when conformance with British Standards became mandatory for all electrical accessories. It would have been up to one of the manufacturers to get them added to BS 5042 (the then standard for BC lampholders, now replaced by BS EN 61184), but presumably no manufacturer thought the cost of the effort worth what they would make in sales. BC plugs can still be found in some other countries which use BC lampholders. Occasionally they can be found for sale in the UK, but strictly it's illegal to sell them now.