insurance completely baffles me too
What I learnt is that it is very important to have lifetime cover, in case your dog develops a long term medical condition that will need treatment or medication for the rest of their lives. If you don't get lifetime cover the insurance company can class long term medical issues as a pre existing condition, and therefore exclude it from any policy you take out in the future.
Have a read around these boards. It is a lot to take in. We went with Axa in the end, lifetime cover with £75 excess. Not claimed anything yet, and touch wood, hope I won't have to for a long time. I have peace of mind knowing it is there, and wouldn't be without it now. It would destroy me if something happened to Bingley and I couldn't afford his treatment.