What are the round-pin wall sockets for?

Round-pin socket

The round-pin electrical sockets are for standing or table lamps and are controlled by one of the twin dimmer switches.

However, no shops seem to sell the round-pin plugs to go in them, but they are available from suppliers on the internet, such as this (no recommendation intended). Thanks to site visitor D Roeder for the link, and for pointing out that for rental properties they would have to have insulation sleeve on the 2 pins.