John McLean wrote...
... it used to be possible to get cyclindrical electrical plug which
bayonetted into the lampholder. A couple of wires could be attached to
that plug and that way it was possible to use power from the light
socket.

Are such things still available?

Or have they been outlawed by some safety regulations?

These bayonet cap adaptors were probably outlawed for safety reasons: -
liable to overloading misuse, no earth available and polarity not
gauranteed.

I haven't seen one available for a long time. In the early days of this
new-fangled "electric", houses often only had wiring for lighting so these
adapters were used for everthing else - including electric irons and room
heaters! More recently BC to two-pin adapters tended to be supplied with
electric shavers so that you could plug in to a desk lamp in a hotel room.
I still have one of these in perfect condition on my desk as I type! Apart
from the lack of earth and indeterminate polarity you need to remember that
lighting circuits are usually fused at 5amp for the entire circuit and so
the amount of power you could draw is very limited. However I do remember
we used BC plugs and sockets to extend strings of christmas tree lights when
I was a kid.

David