Talk about confusing. It really depends on where you'd like to market it. The UK press I asked responded today saying that, typically, the spelling conventions reflect the location of publication, which confirms what Libby said. So a story set in the US but published in the UK would have UK spelling, obviously using US terminology. Sounds like a bit of a cross breed.
But they also said that they personally wouldn't worry about it at all, and it wouldn't affect chances of publication. (So I know where I'll be sending it). 😉