OK lets try to break this down into bite size decisions.
There are basically 2 types of policy. A lifelong policy or an annual policy.
Always,always, always go for the lifelong policy. The difference is this. With a lifelong policy, if you dog gets a recurring or ongoing illness, the insurance will cover it for your dog's lifetime. If you have an annual policy it will only cover it for the current year. The next year it would be seen as a pre-existing condition and NOT be covered. this could be someting a simple as an ear infection or as serious (and expensive) as diabetes which my boy had.
Now lets look at the details of the policy. You need to look at the level of cover. Typically there may be a limit of £5000 per illness per year. Or it may be more or less. Compare this to the monthly cost of the policy. Bear in mind that a serious illness or accident could cost several thousands so make sure that the level is high enough.
Finally decide which insurance company you are going to choose. Base this on people's recommendations from when they have made a claim. My own experience is with pet plan and they are first class, but I am sure others are too.
hope this helps.