The "wirenut" is a quick and dirty way of joining two pieces of wire - it's not a plug. The inside is is a hollow taper that follows the same shape as the outside. It's threaded inside too.
You take two pieces of wire you want to join, strip the ends, and then hold them SIDE BY SIDE with the stripped ends pointing the same way. Twist the stripped ends together, then screw the "wirenut" down over the bare twisted wires. The tapered thread digs in and helps hold the bare ends together as well as being an insulated cover.
In the UK, this sort of thing was in common use 40 odd years ago - you see them occasionally when rewiring a house that still has old rubber cable and ceramic light fittings - but they were made out of pottery. I may be wrong as this isn't my day job but I wouldn't expect any electrical factor in the UK to stock the thing in the picture.
As a possible substitute - how about pushing heavy duty plastic tube over the pole - use a friction fit or glue if need be - cheap, easy to repair / replace and can be cut off if need be. It's a trick used on lots of high peformance kites with carbon fibre spars.
Cheers