Change spelling of favorites to favourites

I cannot seem to find a way to change the US spelling of "favorites" used in that area of the Safari 8 browser to the one the rest of the world uses - "favourites". Am I looking in vain? I minor irritation, I suppose, but it erks me every time I see it. My system and keyboard settings are set to UK English, but Safari does not seem to follow this default.

Any ideas, or am I just being hysterical?