I think Avon's default would be the formal. No idea which he'd use for the situation in "Blake" as he believed the betrayal to be from someone very close indeed. But I can't stand strangers adressing me by first name,e specialy to sell me something, and that's the English equivalent to informal/formal. As Ellynne has pointed out also, Snape is a good substitute or equivalent to Avon, and he uses "Mr." and "Miss" in addressing his students, which other teachers are less likely to do.