My wife is a vet and recommends that you not only look at a lifetime policy but also the amount of cover. Even although companies offer lifelong cover it may be limited to a certain amount of money per condition. If your dog needs continuing treatment throughout its life, or requires an operation then the available funds through the policy may run out quickly.
Our lab is insured through directline on their lifetime policy. He has been pain medication for the last 3 years, which costs over £100 a month. (even with my wife's discount). After we paid the excess they have covered it all.
We're currently looking for cover for our new puppy. Although we got free 4 week insurance with pet plan, they are expensive so were doing the rounds on the insurance websites. Looks like healthy pets is the favourite so far with their gold or platinum insurance. Avila have also given us a decent quote.
Watch out for more than insurance, as well as the excess is £75, you'll also need to pay a percentage of the vets bill too which could be pricey.