With the cat, even if it was an illness that you decided not to treat and prolong their suffering, you could be looking at £1000 just to find out the cause and put to sleep.
The 4k I mentioned earlier, we were around £1000 for initial vet visit to diagnose the (treatable) illness and medication. He then took unwell over the weekend and we had an emergency vet visit (£2k) which again was a treatable illness and wouldn't have been life limiting. His last vet visit where he was ultimately put to sleep was routine as part of his ongoing illness and we decided to let him go at that point. That bill was £1200. Now we could have pulled the heroics and paid more and kept him going for another year or so, but it felt like the right time.
My point is, none of it was heroics or pushing to keep an unwell cat longer than we should have. It was all fairly routine, or responsible ownership (you're not going to leave your cat in pain or unwell over the weekend, and an emergency vet is your only option at that point).
I've always been against pet insurance as it feels like a con, and have always just saved a separate pot for vet fees. We've had some dogs who have had nothing except routine checks and neutering, one dog who had bloat and was an emergency 5k op, and the cat who surprised us all with his expensive last few months. It's the luck of the draw really but given you realistically don't have a lot of years left to pay it, and you're already trying to reduce outgoings, I think I'd err on the side of caution and keep the insurance. Although I do see other posters points about cancelling it, but it just seems like too much of a risk at this point.