With insurance, like pretty well everything else, you generally get what you pay for
There are different ways insurance policies provide cover and you have to take a view as to how fully you want to be covered and of course what you can afford.
When considering what you can afford in premiums you also need to consider whether you can afford to run the risk of cover having been exhausted so that you would have to fork out yourself. It's a bit of a minefield tbh.
Chronic conditions like kidney disease, thyroid, diabetes etc are not uncommon and are pretty expensive due to the amount of ongoing blood tests needed to monitor the conditions and a cat can manage for quite a number of years quite happily with treatment.
Lifetime cover is not that common. Petplan and Axa provide very good cover and, so far, Axa do not I think impose a % related excess and are significantly cheaper than Petplan.
I have just googled using GoCompare for our 9 year old Billy Whiz for lifetime cover and have to say Axa compares well at £23.65.
I was initially interested in Argos as they were coming up with a monthly quote of £20.43 with lifetime cover of £7,000 pa but, on looking at policy summary, they basically don't cover dental to the same extent as Axa and I think most of us know dentals through illness are anything but uncommon!