BMC (British Mountaineering Council). I'm insured by them and it's pretty reasonable considering they actually cover you for the kind of activities where you're more likely to injure yourself (i.e. park/backcountry).
From their website (thebmc.co.uk):
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All types of skiing plus climbing most peaks of up to 6,500m.
What’s covered:
All types of skiing including on-piste, off-piste, cross-country, touring, downhill, telemark, nordic, heli-skiing and ski-mountaineering. Snowboarding can be covered on payment of additional premium. Scottish Winter climbing, ice climbing, alpine climbing. All climbing in Europe and most peaks worldwide up to 6,500m.
What’s not covered:
Climbing in remote or inaccessible regions. Exploratory expeditions and new routes on unclimbed peaks in very remote areas. High-altitude technical climbing over 6,500m, especially in the Greater Ranges (Himalaya, Tien Shan, Karakoram, Andes etc)
I got put onto them by a mate and feel much happier knowing that I'll be covered by them for the kind of skiing that we actually do. Rather than risking it on a cheaper policy that will only cover you on groomers...