I think most people would recommend Petplan. I've taken a chance and have my dogs insured with a new company, Lifetimepetcover.co.uk, so far I've been very happy with them as they've paid out 2 claims, both within a week and their renewals have only gone up by minimal amounts. Insurance can be such a chancy business, I've been caught out when AXA stopped pet insurance and left all it's customers high and dry. I think the musts are to go for a lifetime policy and personally I'd only use a company that specialises in Pet insurance and nothing else. Hope this helps.
Whatever you go for get a life cover policy. I'm with pet plan and covered all my 5 pets with them. i did have a bit of trouble claiming for my oldest as he had a really unusual illness. He was ill for 8 years and in the end they paid all vets and physio fees. My friend works in the business and recommends them. They usually pay. I not had any trouble claiming for the others.