I think it's best to let kids call adults what the adults want to be called, surely?
I do think insisting on surnames is very old-fashioned and Canadian/American. I wasn't allowed to call adults by their first names, as a child, even if they wanted that. My parents were a bit old-fashioned, a bit big on having a large divide between children and adults.
DS1 only calls his teachers Mrs/Miss, and then, only some of them - the janitors etc are Bob or whatever, and the classroom assistant (she helps with a girl who has DS) is just Sue. Which weirds me out a bit, surely if some adults working there are Mrs/Miss, all should be?
Anyway, I'm not Mrs or Miss, I'm Ms, and with a different surname than the DSes. Simplest to just use my first name.