My dog is just coming up to 18months old and I got him at 8 weeks.
I went with petplan lifecover for 4K limit per year. If I could go back in time I would have got the 7k or 12k limit.
He has had many illnesses, investigations, accidents and operations. We have been very unlucky (last one he ate a rag which we didn't know about until he was on the operating table having emergency surgery).
Pet plan paid out around £2500 in the first year and this year we will be at almost the same amount once I claim for his last adventure.
Get a life cover no matter who you go with because in our case we would have so many exclusions right now it wouldn't have been worth it if we had gone with just a yearly one.
Get as much cover as possible. I thought 4K would have been plenty but given what I know now I can see how costs can quickly mount up and my vets are 'cheap' compared to some of my friends.
Petplan advertise that their premiums rise based on your address, breed and age of dog but NOT on claims. Mine went up a very small amount in the second year (much less than my house insurance which I have never claimed on to give you a comparison).
They have also paid out quickly, usually within a couple of weeks.
Whilst I can't tell you about other companies as I haven't used them I cannot fault pet plan.