It really depends on the dog breed, age, £ excess and health for quotes. Heinz 57's are cheaper to insure than some pedigrees because of know issues with the breed.
I have had my dog insured with Tesco since we got her, I think the monthly premium started at £10 a month but now is £40. She is nearly 10 and the insurance has been invaluable, Tesco have always paid out no questions.
Sadly last week we found out she has cancer and we have decided not to put here through treatment as she hates the vets. We will enjoy her last few weeks and make sure she is happy to the end.