There are many threads on this subject at the moment but here is a basic.
I'm with PetPlan for two rabbits. Monthly cost is £20 for both. I have an excess of £65
I only go for Lifetime Policies (this is the only one PetPlan offer for rabbits)
I look for companies that I know have a good reputation (having worked in a vets I know which I will avoid).
e.g. ones that are known to pay out without quibbling costs of hospitalisation, medication fees etc as anything the insurance refuse to pay, you'll be left with.
ones that vets can deal with directly if the bill is too much for me to afford on the spot (e.g. over £1000)
among other things - I'm sure others can help with more detail as I don't have the time to give a full reply