I think I'd find it charming more than anything else.

I'm in a FMA RP right now, and almost all of our players are Americans, but we've just gotten one who's British, and his posts use British spelling. It's noticeable, but not unsettling.

But then, I did my fandom apprenticeship in HP, so UK English (or fannish approximation thereof) feels pretty normal to me anyway.

Also, yay for thinking too much. *waves little flag*