I know how you feel!! My dd is 9 so i haven't come to the week away yet. I feel sick just when she goes on a school day trip!! But when the time comes she will go - they go skiing in her year 6!!!!
So yes, IMO you should let her go. 3 adults from school is enough for that age group - remember they will have the extra adults at the adventure centre for the activities.
"within a school/social setting is quiet and willing to let the adults around her take charge" - even better as you know your dd will take instruction well and won't do anything silly that her peers may be doing.
And i am sure she will LOVE the adventure of it all. Please be careful about putting your own fears onto her otherwise she will grow to be too afraid to do anything. Just think how sad she will be as all her friends are going away for the week and your dd still has to go into school with the year 5's!! Not to mention how they will be talking about it for weeks afterwards.
Good luck! I do feel for you