Petplan does a policy that covers you for a longer period of time (life?) if you need to keep claiming for the same problem. Not sure about the details, you'll need to look it up, (we have our pets covered with them). Think carefully about the cover, not just the initial cost.
The only insurers (as far as I know!) that cover for life are PetPlan, Norwich Union and Marks and Spencer and M&S are the best IMO.
The other cheaper insurers don't cover for life which is not so good if the dog/pet gets an ongoing or very expensive illness.