Don't go for the cheapest - go for Petplan if you can afford it. It is a big upfront expense, but it has been really worth it for me and my two defective cats. I have been paying in for ten years now, but I went back over my statements (I chucked them all in a drawer) after notnowcat1 was run over and broke both his hips and his skull (full recovery, fine now, 6 lives left) and I am still up on the deal by about £200.
Petplan have a 'for life' policy so if your cats keep getting sick with the same thing, such as a recurring bladder infection, they keep paying for treatment, whereas many companies won't pay for repeat infections. They have also never quibbled about paying for tests or treatment - notowcat2 had a bladder stone made of some peculiar material previously unknown to British veterinary science and Petplan paid for tests to be done at - I kid you not - the FBI forensic headquarters in America to find out what it was and stop it recurring.
My cats are both spectacularly accident prone and get seriously sick on a rotation approximately yearly, so I can see that the expensive insurance would not be worth it if you have managed to choose less rubbish cats, but I would wholeheartedly recommend Petplan because you just don't know what the little buggers will get up to next. They even have such a reputation amongst vets for paying out reliably that most vets will claim directly from them if it's going to be a big claim rather than making you pay up front and claim it back yourself (very useful when notnowcat1 racked up £3765 costs in a single day).