I endorse what woofgang and woozer say. Payment for life on long term conditions is vital, some pet insurers will only pay out for a year. I'm with PetPlan for our 3 cats and their claims process is very good. The only drawback (which may be common to other insurers) is that there is not only a £ excess but also a percentage of the total claim as the cats get to old age (e.g. £100 excess + 20% of claim for the 13-year-old) but it's well worth it, Rover's medicines costs three times p.a. what I am paying in premiums. The claim process is easy, you can download a form on-line on www.petplan.co.uk and I have found them really nice to deal with. you should get your insurance really cheaply with young kittens, you will find it helps if they have had their jabs etc and are microchipped (less chance of needing the lost pet section!).
The insurance won't cover routine checkups and vaccinations, you have to still pay for them, but our vet has their own PetPlus arrangement for that, we pay a small monthly fee to the vets which covers the cats' checkups and routine vaccines, and gives us 10% of all other treatments (e.g. flea/worm preparation) and food etc which we might buy there. That really helps - worth checking if your vet has a similar scheme.