Off topic I know but I think even if labelled "Cleaners Use" no company would be taking the risk of exposing their cleaning staff and their vacuum cleaners to 25KV
However there are safety guards in place at depots so the risk is much much lower with far less risk of power surges, also the voltage doesn't vary as much as it would while in service for example if a cleaner was working at Bounds Green depot hoovering a carriage, there is far less risk of a power surge happening then if Joe Bloggs plugged his phone or laptop in while the train was on the move between Euston and Crewe especially as you can get unexpected power surges which is why I said it was irresponsible or even idiotic to suggest use of them after all they are clearly labelled Not For Public Use.
In other words, if you have your device plugged in and you suffer a power surge which fries your device don't bother running to the TOC to complain as it's your own fault.
Not that I am being anti passenger rather the opposite, just merely trying to educate the uneducated why it's not a good idea to use items clearly marked not for public use.
If they were intended for the public then they would be clearly labelled as such, the railways don't label things for fun you know....