Look for lifetime cover that renews the vet fees each year. Some so called lifetime policies allocate a sum of money to each condition but once that is used up there is no more cover for that condition.
Some insurances will consider a cruciate injury in a second limb as a linked condition to the first and not cover it. Nor will they cover any condition arising in the injured leg at a later date such as arthritis.
Petplan are the Rolls Royce of pet insurance companies. They are not cheap but they do not increase premiums if you make a claim. Many companies do.
Like all companies they do increase premiums based on increased running costs, vet fee payouts etc.
Co-op insurance used to do the same as they had the same underwriters but I haven't checked their policy documents recently.
Bought By Many negotiated premiums with established companies on the basis of a large number of applications getting a discount.
They have recently brought out their own policies which offer some advantages over others. This is not a recommendation as I have not used them-( it is only when you claim that you really know what a company is like) but are worth looking at.
https://boughtbymany.com/offers/pet-insurance/landing-dogs-and-cats/ppc/?&DD=pears&gclid=EAIaIQobC!!!!OaygIHg3AIVxUPTCh0KI wmiEAAYASAAEgLFifD_BwE
Avoid any that are cheap- they re cheap for a reason- and any that have links such as underwritten by E & L as there are many reports of them avoiding paying out.