While spelling doesn't read/sound odd (-ou versus -o, -re versus -er), turns of phrase and words (e.g. bollocks, arse, jumper versus sweater) do.

I think "bollocks" would stand out to me more than "arse" or "jumper". It seems much more colloquial. And dialogue that's too full of American slang also stands out to me in TnO fic. I try to be more neutral in dialogue, when I can. It's hard, though!

Is Canadian English like American or British English?

Word usage is more US and spelling is more UK. Just to be consistant. :)