Elly,
As everyone else on here has said, do try to get out at least once a day, so you won't feel that you're in the way. Also, if there's any way to shorten the trip to, say, a week, do so.
I can understand your nervousness. I was very uncomfortable with my in-laws for the first year-and-a-half or so. They had what I see now as understandable misgivings about their son (and he's an only child, to boot) meeting someone from overseas (an American, to boot) on the internet, my visiting, and our marriage five months later (although it was his mother's best friend who married us at the registrar's).
But regardless, you need to relax and be yourself. You are part of the family now, and "hickishness" doesn't play into it. You're supposed to enjoy the holidays, not walk on eggshells. I know that's easier said than done, but please try to keep that in mind.
When I first came over here to meet Andrew, I didn't hear a peep from his parents. So I suggested, after a few weeks, that we have them over to Andrew's place for dinner. His dad was out of town, so just his mom came over. But you better believe I was scrambling to be perfect in appearance, make the perfect but simple Italian meal, etc. She was very standoffish (or so it seemed at the time), and asked me over dinner questions such as did most Americans carry guns, why were we so gungho about the death penalty, etc.--basically, things I'm against). So I politely pointed out that there were close to 300 million Americans, and not everyone was pro-gun, pro-capital punishment, etc. After Andrew talked to her the next day, I asked him what, if anything, she'd said about the evening, and he said, "She said you're too thin." Nice start, eh?
ANYWAY, just be yourself. Andrew and I have spent every Christmas since 2000 but last in the States, and we actually had a nice Christmas with his parents last year. It was a huge difference--I have a very big, extended family, so Christmas Day is noisy and chaotic there--I was amazed at how much Andrew enjoyed being around a big family. Last year, it was just Andrew's parents, Andrew and me (oh, and Millie, the great dane). His parents are the only family he has left except for me and my family. But his mom wanted to make a good impression with ME: She set out an absolutely beautiful table, her best silver, etc., and then Andrew and his dad fell asleep in front of the TV, while she and I had an epic Scrabble battle. (The Gold went to USA, Silver to GBR.)
Don't try too hard. I think, sometimes, that it's easy to take the British reluctance to be open and demonstrative as their disliking you. On the other hand, a lot of Brits think Americans are too open, too affectionate, etc. Regardless, you have nothing to prove--you're as good as they are, and don't allow yourself to feel differently. Just be friendly, don't try to impress. Oh, and a good dose of alcohol certainly won't hurt. Just be sure you don't drink MORE than they do. Get them up to pace.
Suzanne
P.S. I think the turning point in openness with my mother-in-law and me was on September 11, 2001. She had been watching CNN when the report and footage of the first plane hitting the WTC came in. She called us and told us to turn on the TV, and then we saw the second one hit, live. She turned out to be much more empathetic than I'd ever have expected (the plane hitting the Pentagon, she knew, hit literally on my old doorstep). She talked to Andrew on the phone and knew I wished I were in the States, she talked to me (and I was sobbing--something I never imagined in my wildest dreams I'd do in earshot of her), stressing there was no way it was understandable. She came by the same day with flowers and a card saying that all Americans were in her thoughts after an unbelievably tragic day, and brought a candle if we wanted to go to a remembrance ceremony that was being held in Brighton the same night. She was so incredibly compassionate. Even on the one-year anniversary, she did the same thing (brought flowers and a card).
Anyway, just remember that you, your husband and your kids come first. You can find common ground with your in-laws. Just don't push it. And again, plying them with booze can't hurt.
