M&S are also good and have different levels of policy, some of which cover dental.
We use Bought By Many, which is an internet brand of one of the big insurance companies. It's cheaper than Pet Plan and M&S, and don't cover dental, but we've had a couple of claims for other things and they've paid out with no quibble. But whichever policy you choose, you want lifetime cover, so the insurance will pay if they develop something that requires ongoing treatment.
A lot of policies don't cover dental, or only cover injury/disease, not more routine treatment. But I'm not sure if it's worth paying a lot extra just to get dental cover, because it doesn't usually cost that much in the great scheme of things, so it's probably no more expensive to get a cheaper policy and pay for any dental work as needed.
For example I have a 15 YO cat who's had quite a lot of dental work over the years as she has a condition that the vet described as being like 'allergic to her own teeth' so I think she's had 3 GAs and significant dental work and it's 'only' been about £3-400 each time, which I know is a lot of money, but nowhere near the amounts that the insurance has had to pay following road accidents or years of investigations, treatment, ops and even a radioisotope procedure if they develop an overactive thyroid.