I have a number of these (purchased from ebay), since more and more lamps available in the UK are now supplied with an Edison screw fitting.
One thing to bear in mind when using these converters is that a correctly wired Edison screw receptacle will have the Live connected to the central connector and the screw thread (which may be partially accessible) connected to Neutral (thereby minimising the risk of electric shock).
When using a device converting a bayonet connector to an Edison screw connector – the polarity of the Edison screw connections may be transposed (depending on how the bayonet connector is made); which could result in the accessible screw connection being Live.
My advice is to take extra care when using these converters.