I do, I've got a policy with M&S, its about £11 a month, not only due to the FIV, but the fact that Max is nearly 9 as well.
I'm not sure of the merit of the insurance though, to be honest, as any illness which they could attribute to the FIV would be excluded. Dental work is also exluded.
It does however cover for any eventualities like breaking a leg or anything like that.