Lettie - you're very lucky that your premiums haven't increased, and also that you haven't had anything to claim for.
I would still recommend moving, though, as Axa has shown itself to be an unscrupulous company (I'm one of the others with ridiculous rises). I'm currently waiting to hear what the outcome from the Financial Ombudsman investigation is, but in the meantime I would suggest everyone move elsewhere. If a company thinks that it's fair to increase your premium 610% for any reason whatsoever then it should be boycotted! If it can do this to me then it can do it to anyone else. They've admitted to me that the premiums are based on a whole host of things, including your claims history and potential claims future, and from what PetsPlan customers have been told, they don't do this. My vet also said that this should not be the case, as you shouldn't be penalised for having an ill dog (if you paid the premium prior to them getting ill).
To the others who are still with Axa, I would also shop around and look elsewhere. You should be able to cancel your policies without any cancellation penalties. Even if they don't increase your premium now, who knows what they might do in the future. If you've never claimed from then and your dogs have no pre-existing conditions, then I can see no reason whatsoever to stay with Axa!!