Hi,
I'm a vet...haven't got much experience of looking into insurance from a pet owner's point of view...but in terms of what they cover....
some of the variation in covers will be in amount per year (in all for various conditions), or amount per condition/per year. So some may only be up to £3000 for the year, others may be up to £10,000 or even more. Some will only pay out that total once in a year, others would pay that up each condition.
the other big difference is companies that won't pay out on pre existing or ongoing conditions. so your dog may have an ongoing disease such as diabetes and you only be able to claim for it for one year. or conditions that recur, skin allergies, ear problems etc.
One other thing, is the difference in 'things' you can claim for, the better policies often allow you to claim for behaviorists, physio, hydrotherapy etc, although policies wont.
Rough costing for some things, referral for staging and chemotherapy for lymphoma - £6000-7000. Referral for just an MRI scan for spinal problems, £1500 - 2000 (and then another £3000-4000 if surgery required). Things that can be dealt with in house rather than referral can still be expensive, £700 for drip, bloods, ex lap surgery (eg, dog swallowed a foreign body). Medication for atopy (allergic skin disease) can be expensive, for a 40kg dog, cyclavance (one medication option) could be about £200 per month. Immunotherapy vaccine, £200 for about 10 doses (prob required lifelong).
Those are just some rough costings. Generally most people don't have that kind of money at the drop of a hat. If your dog swallowed something tomorrow and needed surgery, would you have £700+ to spend? That's the kind of things you want it to cover for.
If you are going to shop around/switch insurance, do it whilst your dog is young and doesn't have preexisiting problems. If you wait until it's older you kind of end up tied to companies as they will often exclude things you have had before (been to the vet for, even if you haven't claimed).