Just called BUPA to renew the annual travel insurance policy that was taken out by my previous employer. I asked what the cover was for mountain biking and they said: Helmet, knee and arm pads must be worn at all times. Only on "an authorised trail" and only on a pre-booked (sic) guided ride with qualified supervision. No high altitude and no "extreme terrain" although they couldn't define exactly what this meant. So not worth the paper its written on then.
Any more realistic recommendations for an annual travel policy?
Check out Snowcard.
Another vote for snowcard, but you'll find you will need to select level 3 that includes downhill and off-piste riding to cover yourself.
Those conditions you mention are all pretty standard I've found, except for the knee and arm pads bit.
The crazyist one I found was one company that said "no cycling down vertical slopes" - what the hell does that mean? I phoned them up and even the company couldn't tell me what it meant, lol.