I have just bought some insurance with snowcard.co.uk for a trip to Austria cost me 52 quid but thats kit and injury insurance for nearly a month. I also sorted out the new version of the E111 form (European health care card). If you have a look on www.ukriversguidebooks.co.uk in the forums there is at least 2 threads on personalinsurance, 1 of which I stated asking the same question as yourself.
I have taken it that you are not staying in the UK and youi may find that those with PAX etc could already have cover. I also remember seeing that if it goes on detail/orders then you are covered under the Army system but cant recall the exact details for that and better to have your own cover anyway. Hope this helps