Alot of people go with your common insurance companies for shooting, so BASC, NGO, etc. But actually, as far as I am aware, some home insurance policies allow you to add shooting cover to your policy, so explore that option too.
I'm not part of BASC, but I think (someone may correct me, or you can look on there website) they are an all members covered organisation. This means that you just join BASC and automatically have insurance.
Although, dont quote me on that. Like I say, someone may offer more insight.
Atb, BB
Edited to say: It may not be that the cheapest/dearest policy