Have been with Kennel Club Pet Insurance (currently underwritten by Agria) for some 12 years.
Have always found them to be prompt to settle claims and with no quibble. They also do not have the many opt out clauses of some pet insurers.
On the downside annual premiums are now increasing rapidly but accept that this is due in part to the escalating vets fees. It is unfortunate that this coincides with the pets becoming older as well as their now retired and pensioner owners!