I'm with PetPlan. Probably costs a bit more than other policies but never been any quibble and it covers dental and whole of life treatment, so if the cat gets a chronic illness it is covered for life.
The vets like it too (not sure if they get commission) but it means the vet instantly knows what is covered.
The limit of normal cover is around £4000 a year, which from my experience is enough to cover a cancer tumour removal and some.
This is the second cat I've had insured under Pet Plan and honestly I cannot recommend them highly enough. Sam 7 spent his final days in a specialist pet hospital and had an MRI, which came to around £2500. I only had to pay £85.
The only thing to be wary about is when your renewal month is, because if the cat is having treatment around renewal date you might get caught with paying the excess twice.