I always advise a lifetime policy, with as much cover as you can afford.
Personally, I always go with PetPlan because, although premiums do go up, they don’t shoot up drastically and not in relation to claims. I also prefer them because every vet we’ve had (including specialist vet hospitals) have been happy to claim from them direct. These days the vets do it electronically and for the last two claims, I had the money in my bank within 48 hours. Bought By Many seems popular on here though. Not sure I would go with John Lewis, as I doubt many vets would do direct claims with them and it really is a big help and one less thing to worry about when your dog has emergency, ongoing or complex treatment.
My reason for advising lifetime cover comes from having had various rescue dogs that have developed chronic conditions, which needed ongoing medication. My current dog is 16 and on two different meds, which I claim back the cost for, alongside his monthly monitoring appointments, every 6 months. Even taking into account the increase in excess etc, that you get with geriatric dogs, we are still better off with, than without the insurance.
My reason for highest cover you can afford comes from the dog we lost last year. He developed a horrendous auto-immune condition that cost over £20,000 to treat in 18 months. If we had had lower cover, we wouldn’t have been able to afford his treatment. Also the specialist vet hospital were happy to claim direct, so we didn’t have to pay a penny upfront for his 7 day ICU stay and it was such a relief not to have to even consider the cost when agreeing to his treatment. (His premiums didn’t increase any more than our other dog’s after the first year of mega claims either. Other dog is now 16 and we pay £75 a month for him - he’s a large Lurcher.)