Vet plans and pet insurance are totally different things.
Vet plans normally include things like free check-ups, flea and worm treatments and vaccines for a regular monthly amount - normally around £10-12 per cat with a multi-cat discount. Some also offer a discount on spay and neuter.
Insurance covers you if your cat gets sick, injured, lost, hit by a car etc. You'll need to pay an excess (anywhere between £60-150 normally) for each claim as well as your monthly insurance amount.
The more cover you go for (in terms of bills) the higher your monthly payment but you can bring that down by choosing to pay a higher excess for each claim.
You also want to make sure you have lifetime cover so that if, for example, your cat has a stomach bug when they're one, you can claim again when they have another stomach bug aged six.
As they get older (as in, elderly - 10+) you may find you have to pay a percentage of their vets bills on top of the excess too. Normally around 20% or so but it depends on your policy.
We have Tesco lifetime cover for our three - it covers up to 15k of bills per year, per cat and costs £30 a month. They're seven, six and two
we've had to claim once and they paid out immediately and were very easy to deal with.