This thread has been running for years and years! I don't believe at all that "hey" goes back to a native American greeting. It's been used in British English for a very long time to attract someone's attention dramatically, as in "Hey, you there!" or "Hey, stop that!" It's used mostly towards strangers; otherwise one would call them by name. It's certainly nothing to do with the "silent (??) language of lovers" in Britain. It can be aggressive, and is never used to address someone in an email.
Hi is definitely an import from America, still resisted by some who don't wish the English language to become americanised, and is an informal greeting equivalent to "hello". It's become accepted as a greeting at the beginning of an email, instead of the more formal "Dear...".

"I was sat" is horrible but I've even heard BBC correspondents use it. Here in France there's the Academie Française to protect the language; but in the UK so many people have never learned English grammar at school. And now that there's something called "international English" - which I come across a great deal in the course of my work - the richness and precision of English is quickly going down the pan. The French resent the fact that English is the international language these days; I tell them be glad that it isn't their language which is being chewed up and spat out. Much of the problem comes from slang American I'm afraid, e.g. the use of "like" for "as if" or "as though". (An educated American friend confirms this.) Like is used before a noun: "He looks like a horse." Before a verb it's as if or as though: So "It looks like he's going to be late." doesn't sound right. We are losing much richness: the word "that" is used for "who" and for "which"; the word "which" is disappearing altogether in international speak.

Well, I'm a foreigner in France at the moment, but I'm not an expatriate because I'm still officially resident in Britain. The French are unlikely to have a headline beginning "Frogs..." but they might well have one beginning "Les Rosbifs...". One might well see a headline "Scots do well..." or "Welsh..." or "Northern Irish..." but "British do well..." sounds a bid odd and old-fashioned. "Great Britain does well..." is a bit of a mouthful and so would probably get reduced to "GB does well..." but some people don't like the term "GB" any more than others like "Brits" - so who to please?!

I think hey is a horrible greeting, since in English English there's an implied exclamation mark, as with oi! As in "Hey you! Stop that!" So it's another importation from the States, as hi was originally, although hi is very widely used in Great Britain now.