Early domestic electrification of homes in the UK principally consisted of electric lights.  There were few, if any, wall sockets which were reserved for the heavy duty appliances.

Appliance connections were also two-wire, (live and neutral) the earth being derived from a literal Earth Point, or an attachment to the lead water pipes, when needed.

It wasn't until the 1960's that legislation came in to enforce earth connection on all appliances (and hence ratify the three-pin socket and plug system).

This image show a 2-way connector, the top of which is plugged into the light socket.  The bulb is then replaced in the lower socket and the appliance run off the side socket.  The existence of a shade ring implies that this was to be permanently placed.

I must admit, I have used this type of plug (which is now illegal in the UK), to power a photographic lighting set up as it is so useful.