Thank you so much for the helpful responses!
Noreneh - I'm looking into West Dean College, so we would most likely live in Chichester. My daughter currently has a difficult time adjusting to new environments, but a potential move is a few years off, so maybe she'll do a bit of maturing (if not, we won't go). A nickname is an excellent idea - we'd only thought of using her middle name.
HeadingBackHome - Good luck on the transition back to the States! I'm from the Midwest, and the one student we had with an English accent was treated like a rock star because he was so interesting. I hope you have nothing to worry about!
Smashley - My daughter is also an only child used to a lot of adult company. She tends to not care what others think of her (not in an arrogant way, she's friendly, but confident), so I'm afraid she won't really try to make friends. Either way, it's a few years off, so who knows. I'm glad your son's situation is improving!
Again, thank you so much for the help - this forum is a wonderful resource!