Thread sizes below 1/4 confusing

OP is AU, correct?

Not sure his light switch plates are the same as ours.
Yup Looks like Queensland.
Must be some ironmongers or specialty houses with Unified/USS/Imperial/Whitworth & British Standard fasteners. But maybe no more convenient than if he can buy the correct ones from a specialty house here.
Look at the trouble even we have getting fasteners that used to be common here. :(