Geoffrey:
English english, v American english. Easy, who is talking or thinking? They will use their natural tongue, unless for effect upon the person they are talking to. Of course the narrator, if you have one will also have a voice and will choose the appropriate language--see, it's not you deciding, it is the character.
Absolutely. Since I'm British, the majority of my stories are written in English English. But I've written quite a few in American English - 'First Date', 'Sixteenth Birthday Blues', 'The Domestic Pacifier', etc - and it always seems to come quite easily. And so far, I'm glad to say, I've not been told that any of these stories don't feel authentically Stateside.
I think it's partly because I enjoy writing pastiche, adopting someone else's voice - not only when I'm pastiching specific writers, such as Jane Austen, Charlotte Bronte, Wilde, Huxley and so on, but writing first-person as a specific character, say a bratty teenage girl ('Not Her Best Day') or, if it's a period story, modifying the style and vocabulary to fit the period. It's a challenge, and for me that's always fun.