we use pet plan, they are a bit more expensive, but we get a multi pet discount (we have 2 dogs and a cat) and we have found them to be very good.
we currently have two claims waiting for Annie and they have been very efficient with them so far.
Molly has ongoing medication, which we have to just pay the excess on each year, but including the payments and excess we pay alot less than if we did not have insurance. Alot of insurance companies will only pay for a condition for 12 months and not for life.
I have heard that M&S is good, but have no experience myself.
hth