I looked into insurance but decided against it on cost grounds. I put £30 aside per month (I have seven piggies atm) and that has been fine so far. I ought to increase it a bit.
For dogs - I read the cover outline for the Tesco accident insurance, and thought that the only thing it would cover is IF the only thing that happened to the dog was an accident AND it lived, THEN it would pay out. But for nothing else. Not worth it. I would defo follow Amanda's advice re getting cover for long standing conditions, including things like diabetes.