1. Look for an established company. Once anything has happened to your dog it would be excluded from a new policy so you need to pick a company and stick with them. A 'lifetime' policy means they will go on paying for a chronic condition, it does NOT mean the price won't rise. The price always goes up as the dog age as the risk of a claim rises.
2. Vets are not supposed to recommend particular companies. This isn't an endorsement, but conversation between stalkers on a forum. I know a number of vets who have their own pets insured and all of them use PetPlan. BASC dog insurance is via PetPlan, covers working dogs and is at a discounted rate. NFU are great to deal with. Take heed of Buchan's letter recommendation and if you haven't heard of the company ask who the underwriters are!
3. Cheapest is NEVER the best. Read ALL the small print. I don't think I have ever had insurance fail to pay when there was a justified claim, the problems stem from people not reading and understanding the small print. Some require claims to be submitted by certain dates, or even pre-approval for some non urgent procedures. I know a few people who have changed companies and had claims refused for arthritis because the puppy was a bit lame 10 years ago. Some policies require things like vaccines to be up to date, and they may even try to wiggle out of claims not related to infectious disease.
4. Comparison cost sites are a really, really, really, really bad way of choosing pet insurance.