Congrats. Great trade and it gives hope to me for using my inland marriott to snag hilton head in the summer. Were you the family that was thinking about Branson as an alternative to Hilton Head?
About the flights. I can't speak about flying into Hilton Head from London, or wherever you are but you should understand outside of Charlotte, North Carolina, there are no real airports in the Carolinas. There are small regional airports, but that is it. I can't immagine the Hilton Head airport being much of anything. I think most people that fly into Hilton Head from the states, fly into Savannah and drive the hour or so to the island. Everything in the Carolinas is a "drive to" destination for most of the Eastern and Mid-Western United States. Not many people fly as there really aren't big airports near Hilton Head or Myrtle Beach and you ABSOLUTELY NEED a car at either destination. NOTHING much is within walking distance to the resorts and outside of biking, you will need some kind of transportation on the island.
I don't know how else to describe Hilton Head or Myrtle Beach other than spread out and Hilton Head really doesn't have any attractions to speak of outside of some small shops and restaurants. There is a nice outlet mall just off the island, and you can drive to Savannah, but other than that, there isn't much on the island. Everyone vacations different but Hilton Head is where people GOLF, go out to eat, relax on the beach and wander through a few mom and pop shops sprinkled throughout the island. There is always biking and Hilton Head is unique in that you can bike to most things, but it could be more than 5 miles one way to get to most things. One thing you can bank on is very oppressive heat in July, so biking may not always be an option.
You might be better served flying into Charlotte, NC (are there direct flights to/from Europe), renting a car and driving to Hilton Head. It will be about a four hour drive. Savannah is about an hour drive but that's a small airport and I'm sure you will have a connecting flight or two before you get there.
And if you are planning on staying two weeks in Hilton Head and getting a back to back week trade via II to accomplish this, I would have a serious "B plan". I think you should play the lottery, that is how luck you were to get a Hilton Head Island July trade, not so sure you can count on another July trade, especially for a specific week -- but it MAY happen.
So happy for you to get this trade. Have fun, I love Hilton Head, but don't expect much action on the island and do plan on having a car for the entire week.