Are electrical screws a standard diameter?
There's no reason it would not be a locally standard size (given a British location based on the label, presumably metric or pre-metric British Standard threading) so some basic measurements (no calipers required, if you are willing to use a ruler carefully and buy 2 or 3 nuts, one of which will be the right one) and a trip to the hardware store should find a nut to fit.
You might also try putting fine net mesh over a vacuum cleaner inlet and vacuuming for the nut, first.