Yes, heathfield is correct. The commercial sale of these bayonet connectors was made illegal some years ago as foolish people used to try plugging high wattage appliances into the lighting circuit with them.
At the time, most houses had a 5 amp rewirable fuse in the consumer unit, which meant that in theory, the lighting circuit could handle up to 1200 watts. However, this was always a theoretical figure as invariably, people would use their lights at night causing the circuit to be already loaded. While 1200 watts sound a lot, it's only twelve 100 watt lightbulbs after all.
Overloading a lighting circuit in this manner has caused more than one house fire in its time and this was the reason they were banned.
I well remember as a lad that the Dansette record-player my grandmother used was wired with flat, two core flex (like bell wire) into one of these bayonet connectors and plugged into the bulb holder in the room ceiling. Those were the days!
I wouldn't bother looking for one of them as they're potentially dangerous.