If they're youngish and healthy when you get it, the pet insurance is definitely, definitely worth it. If they have any pre-existing conditions (which a kitten shouldn't have), then it's probably not worth it. We've had really good experiences with pet health insurance. A friend had a less positive experience. She adopted a middle-aged rescue dog with a pre-existing condition, and every single thing that went wrong with that dog's health went back to the pre-existing condition, so the company wouldn't pay.
I have no advice as to which company to buy the insurance from, though, as we're not currently in the UK. Oh, and I can't speak for the UK, but here preventative care is not covered. i.e. Yearly check-ups, vaccinations, teeth cleaning. Only when they actually get ill. Our cats are indoor only, so they get a small discount. UK insurance carriers may or may not offer the same deal.
It's the peace of mind more than anything else that you're buying. For our current two cats, we've never had to use the insurance. For our previous cat, she got a very expensive illness and eventually we had to have her put to sleep. But if she hadn't been insured, we probably would have explored the PTS option a lot earlier, because her care would've been eating into our savings. With the cat we had before her, we didn't get insurance (she was 9 when DH adopted her, and it wouldn't have been practical). We had a rainy day fund that was supposed to be in case the cat got ill, but inevitably we raided it for other purposes.