I'm with NFU Mutual and have been since William was a puppy. They're not the cheapest, but their lifetime cover means the whole life of the dog and they also cover medical conditions for the lifetime of the dog. Some policies will only pay for a condition for 1 year or up to a monetary limit and the owner has to pay after that.
William and Louis are both insured with them and so is my friend's 12 year old Golden Retriever, who has back problems. She has regular hydrotherapy and acupuncture to help keep her mobile and pain free, and has been claiming for them, plus medication, since diagnosis of the back problem. Her premiums are about £100 per month now and excess is £200 for new conditions. But that's not bad considering that my friend claims at least £1,500 per year for the hydrotherapy and acupuncture, with no excess to pay on it since the first claim 4 years ago. So she's claiming more than she pays and could still claim for any new conditions too.