I'm Australian and we tend to use both UK and US English. I think we're generally more UK English leaning, though. I was certainly taught to default to UK during school. This means that when I write fills I keep the general text UK and make the dialogue US. It's probably a little jarring, but I kind of have fun with it.

Weirdly, this has been most relevant for arse vs. ass.

Kinkmeme, I guess.