raises hand

Hey, I'm originally of British origin. I can indeed confirm that the language Harry uses has made me wince a little. This hasn't happened in the last few chapters, since we've been hearing from harry!Mort rather than Harry, and mind-dumps don't respect style, but

"I'm in Mary's Place, Professor, in Diagon Alley. Going to the restroom actually. What's wrong?"

-contains the word "restroom", which no speaker of British English would <i>ever</i> use in that context, and the question "What's wrong?" is a little aggressive. I would suggest something like

"I'm in Mary's Place, Professor, in Diagon Alley. Ah, I'm actually just going to the bathroom - is there something wrong?"