When we (SSF) went to France a couple of years ago, we tried to find an insurance that covered expedition caving. An SSF member thought that he had found such an insurance company (Gouda reseförsäkringar), and several of us signed for it. They explicitly claimed to cover "cave descending" and "cave walking(?)".
However, a member that was more sceptical than the rest of us, and explained in more detail what we were going to do, and what would take to organise a rescue, and then he got another answer: "No, our insurnace does not cover THAT."
You can check Gouda, but be sure to find out what their claims really mean when it comes to rescue. If the insurance cover e.g. expedition mountaineering and climbing, it may include caving...but still, cave rescue is a bit trickier...
Afterwards, we got the tip that it would be better to enjoy a british caving club, but I don't know if it's that easy (to enjoy and if it solves the problem).
Kalle G