You might try Kingston Grammar School. Their playing fields and boat club are just the other side of the river. Its walkable, or even better if you can get a boat accross the river.
They've definately had helicopters in before, and being a private school are always looking at ways to raise money...
Don't know who would be best to speak to. There is a full time fundraiser I gather. Or the burser, or perhaps the chap who is in charge of the CCF...!