(though I freely admit I'd never heard the word 'timeously' and don't have the first clue what it means!)
It doesn't mean anything more startling than 'early as possible'. It worked its way into my daily vocab after years of typing "Dear Sir, We'd be obliged if you could send us your payment timeously, otherwise we'll sue your butt off." Or words to that effect.
US -v UK authors: I don't think you can ever immerse yourself completely in US language/writing styles if you're a UK author and vice versa. I've always been in the first category you mention. I try to make it as US as possible, partly because I feel if I'm dealing with a US show and characters I owe it to them and partly because I enjoy the American language and always have done, so it's always been a joy to discover the differences between the two.
I can understand why UK readers would be irked by Harry Potter celebrating Thanksgiving, just as US readers would be irked by, oh say, Lois and Clark celebrating Guy Fawkes Night (unless, of course, there was a good plot reason for it that acknowledged it was something out of the ordinary for them to be doing) - probably justifiably in the extreme cases. But I think a reader at the other end of the spectrum, like those you mention, who won't read a story if its not 100% UK/US is missing out on a heck of a lot of good reading!
LabRat
Athos: If you'd told us what you were doing, we might have been able to plan this properly. Aramis: Yes, sorry. Athos: No, no, by all means, let's keep things suicidal.
