In reply to lep:
I have several life insurance policies (for endowment mortgages) and none of them increased premiums due to my climbing. Norwich Union, Standard Life, and Commercial Union I think. Of course, their policies may have changed since then, but keep shopping around and you'll find something, the market's quite competitive at the moment I think.
The important thing though is to be honest. If you give them untrue information (eg by saying you only do easy stuff in the UK, if you *ever* climb abroad) then the policy is invalidated, whether or not you come to grief while climbing. Not worth the risk.