
Ideally you should buy an insurance that gives you cover for life - so if your dog should develop a condition that is ongoing the insurance company will pay vets bills for that condition for the rest of the dog's life as long as you continue to pay the premiums. Some insurances only provide cover for a year and then exclude that condition at the next renewal time. Insurance won't cover any condition that is pre-existing so even if you changed your insurance at the end of the year anything your dog had suffered from during the insurance period wouldn't be covered by a new insurance provider.
Vets bills can soon add up if your dog is unlucky enough to need any form of extended treatment so it is worth your while to take out the highest level of cover you can afford.
I have one dog insured with Marks and Spencer's and have so far been very pleased with the cover they have provided and the speed of payment of a claim. I have also used Argos and was pleased with their cover as well although didn't renew with them this year as the premium had trebled in price.
You need to have a good look at the small print before taking out a policy, as with any insurance.