With the 12 month policies you have the amount insured for just the 12 months, so if you had to claim for an illness that would be ongoing, like diabetes, arthritis, kidney, colitis, etc then after the 12 months were up the insurance company wouldn't pay out for anything to do with that illness again. Most companies that offer lifetime cover will also cover the cost of prescription foods that the animal may need to be on for life. With a 12 month policy you would have to pay the cost for these aswell as any costs for the treatment no longer covered after your first year.
Most life cover policies come into their own as the pet gets older as this is when they are more likely to develop chronic illnesses but some pets will develop chronic conditions at an early age. All this is worth taking into account as once your pet develops a condition, if you then switched insurer they would exclude that condition anyway. Is it worth the risk?
Another thing to bear in mind is that some of the cheaper life cover companies will either increase the excess or make you pay a percentage of the cost of treatment on top of the excess when the pet reaches a certain age. This can be quite costly if they need expensive treatment.
It is worth comparing these things and asking which is better in the long run.
Also, without sounding harsh, if you are expecting to feed and insure a dog on £20 per month then you should maybe have researched things more before getting the puppy. You have not factored in to your monthly costs the price of routine preventative treatments that are needed like vaccination, worming, flea treatment and of course the cost of neuturing at 6 months. The cost of treatment for a condition no longer covered by the insurance would also be alot more than the difference in premiums.
Insurance companies don't take individual animals breeding into account either. They use the breed as a whole based on what that breed is more susceptible to.