its very hard to get a cheap cover that's any good for your dog, even worse if its a pedigree. i have to change policy every year because the policy starts cheap and then on renewal doubles or trebles and they hope you don't notice! At the moment pet-insurance .co .uk is right for me but read the options and small print and the excesses!
I use animal friends. They have good lifetime cover options for my Great Dane. They donate money to animal charities too
http://www.animalfriends.org.uk/
PetPlan are also very good, but too pricey for a Great Dane.