£15k seems a lot - you'd have to think about quality of life, especially for an elderly cat, if the bill was that high!
I haven't used either of those companies (although I've heard Animal Friends doesn't have the best reputation) but I have my 17-year old tabby insured with Tesco. It's limited to £3k per year or condition but there's only the excess to pay, no percentage of the final bill which a lot of insurers now charge for older cats. And I've found them really easy and helpful to deal with.