Definitely shop around fairy100. Insurers will offer different products, and not all require a history, particularly with rescue cats. Use comparison sites, and check the level of cover very carefully to ensure you get a level of cover which is suitable.
As with Paula, talk to the insurers if in doubt. We use Healthy Pets for our 3. Two of them are semi feral, and we can't get them to the vets without a major struggle. They're covered for anything other than a disease resulting from lack of vaccinations. Our Insurance is still less then £14.00 per month for the two, and Moray who is older (and much more amenable to vet visits) is not much more, even though we had to make a claim this summer. All are rescue cats.