In reply to lumu_tit:
As per Simon C. I was looking into it recently, and I think the AAC only covers you for £10,000 of medical expenses. Which in the States will barely get you an ambulance ride to hospital, let alone any treatment.
I also spoke to Virgin about adding rock climbing onto a general travel insurance policy... They will include rock climbing but exclude mountaineering (and were unable to give me a clear definition of the difference) and the cost per day of adding on rock climbing was £14. So the BMC insurance worked out cheaper for our 15 day trip.