It's a bit different to most types of insurance as you want to buy the best policy for the long term right from off as can't always shop around each renewal due to pre-existing conditions which other posters explained, it doesn't matter if not claimed for and they request full records of any vet been to on first claim. They also tend to group pre-existing so a minor lump = all types of lumps incl cancer, that puppy diarrhoea= all bowel conditions. Some minor things maybe reconsidered and added back after a period has passed without treatment but isn't guaranteed.
The cost difference is because many lure you in cheap on first year and while dogs young, but may whack up premiums post claims or when dog gets old as risk is then higher but it's often when need the coverage most & have you over a barrel. Some also quibble claims to avoid paying out so you may end up left with the vet bill or do it slowly and bear in mind that vets won't claim direct if they know will likely be left unpaid so you maybe expected to pay upfront.
Some don't penalise individual for claims others do so have to consider if it's worthwhile making claims when premium/possibly excess may increase for rest of lifetime.
Some cover dentals and others only if accident, these can be expensive my dogs last was about £500 and some just have poor teeth no matter what you try. Do they cover behaviourists or complimentary therapies, physio, hydrotherapy, nutritionist, acupuncture etc.
Read the policy details carefully as many get caught out believing are covered for xyz and aren't because it actually contains limitations within. Is it covering conditions for the dogs lifetime and the vet bill pot renews annually or is it a limited that will only pay £X per condition over lifetime which isn't as useful.
These days need a minimum £7k vet cover per year on lifetime policy, ideally as much as can afford as it doesn't increase so in 10yrs time how far will it stretch, probably seem not enough in even less time given current inflation of vet bills/meds etc.
I insured mine with petplan which are more expensive but had good reputation & longevity as some had recently left pet insurance market altogether or underwriters changed & those with policies can end up shafted. They pay quickly for claims and all vets & specialist referrals would take direct claims so I've only had to find the excess. Cover dentals ( if keep to terms) & complimentary therapy. Can claim for everything without worry of effecting premiums. They covered my dog up till passed at 17yrs and premiums will always rise as costs/risks increase but were steady. Paid 20% bill on top of excess over 10yrs but i'd rather keep premiums affordable and pay for what I'm using. Also cover stem cell therapy for arthritis which gave her much better quality life and I'd never have afforded otherwise.