I can only speak from my experience as a child, but I'll share that perspective since you have children of your own. My father, a law professor at Notre Dame, took a year's sabbatical in Oxford when I was four years old (1960-61) to research a book he was working on. We had four children, and a fifth was born over there. Obviously, a four year old isn't going to mind much where he lives, and will adapt easily to the changing culture.
Ten years later, we took two years in Oxford. My father had one year of sabbatical, and a year teaching in the law school's overseas program. Again, I can't speak much about the financial aspects of the trip. By this time, there were seven children, and the house we stayed in was pretty small, so that will tell you some things! On the other hand, the cost of living was lower there than in the US at that time.
I left the US at the end of 7th grade. This time, there were more challenges for me personally. When I got to Oxford (a month before school was to start) and we got sorted in our house, I took off to look around. I went into a newsagent's and struck up a conversation with Bruce, the older kid behind the counter. (He became a lifelong friend.) He told me about his school, and said I would like to go there. As it turned out, they were looking for someone to deliver papers as well. I must say my parents were quite surprised when I came home three hours after moving in, telling them that I had a paper route (they would say "paper round") and a lead on a school to go to! Dad went and had a chat with Bruce. One of my brothers and I wound up going to his school. So, I guess one of the things I could suggest is to improvise a bit, trust to providence, and trust in children's innate ability to connect emotionally with one another across cultural boundaries.
On the other hand, your children (especially your older ones) will have more than the usual challenges to fitting in. I started school in second form. The kids had already had a year of algebra, a year of French, and a year of Latin. I had a gift for languages, and my mother taught French and knew Latin very well, so those weren't much of a difficulty. However, I had difficulty with algebra and wasn't the sort to ask for help when I needed it. Then, when we got back, I was a high school sophomore, and in second year algebra again. The upshot of all that was that I never really did well in algebra until I took a remedial class in college.
If you have a child who will be 12 on the way out and 14 coming home, he will have gotten a lot of mixed messages about social rules as well. This is a great way to learn that social rules aren't as important as they often appear, but the effect of the confusion shouldn't be minimized as well. Kids learn a lot of their ways to interact with one another during those two years, and having to adjust a lot of them during that time shouldn't be underestimated as a difficulty. The usual adolescent insecurities get magnified, and parents may have to do a bit of extra work to help a child get over them.
When people ask me if I liked living in England, my response is generally "Did you like being 14?" But all kidding aside, as a grownup, I can say that this was a great life experience for me. Exposure to a different culture at a young age gives me a deeper understanding of what does and doesn't matter about my own. Also, I was exposed to an entirely different educational system, which educated me on some of the strengths and weaknesses of our own. Finally, I'm probably the only Oxford United fan in the entire United States. :)