I'd really recommend Pet Protect, as Ive had my pets insured with them for years and they have been fantastic about paying out when things have gone wrong. No hoops to jump through to get your money back (like some companies), and really sympathetic letter when my cat Brian died.
Im looking into it at the mo for when our 6 weeks insurance runs out, and they seemed to be cheapest too, for the amount of cover you get.
Make sure you get lifetime cover - ie, that they will cover you for the lifetime of the animal. A lot of cheaper ones will stop paying after a limited time if you pet has a lifetime illness.