You tend to get what you pay for; the cheaper policies/companies are less likely to pay out when needed, and you will probably have to pay the vet yourself and then reclaim from your insurance company. Make sure you buy a 'Lifetime' policy not a 12-month one.
I looked for 'lifetime' policies, then took the cheapest which happened to be The Kennel Club. My dog is now 2 and I haven't made any claims. I am considering stopping it and putting the money in a bank account instead.