It's allowable, but not recommended. Consider a hardwired USB supply (available so they fit into a lightswitch), or labelling the socket to indicate 'USB CHARGERS ONLY' or similar.

If someone plugs in a vacuum cleaner or kettle, it might trip a breaker.

What sort of USB device are you looking to run? You could get one of these PDL jobbies

https://www.pdl.co.nz/search?q=usb&datasource=products

The work sparky told me that if the outlet is installed for a specific purpose and is positioned so it would be really hard to use for something else, then it is not a general purpose outlet so is fine to go there, which is why rangehoods, extractor fans, HRVs and other stuff is all fine on lighting with an outlet which is its point of isolation. Still counted as adding to it so supposed to have an RCD added tho. Not sure how the HRV people justify getting away with not adding one when hacking into a light circuit to put their glorified fans in.

Many thanks for the help. Both ideas would be suitable. A power socket could be up at ceiling level. Will leave the choice to the electrician.

Ken