Pet Plan is generally considered very good.
I'll tell you about my experiences with pet insurance and you can decide if it's worth having ....
Four years ago we had 3 dogs and 2 cats. Cats and 2 dogs insured with Direct Line. Premiums went up each year as pets for older. Got to a point where they wanted about £250 pcm for all 4 and I thought, this is just ridiculous. So I cancelled insurance for the cats (good call - never really claimed for anything, sadly neither no longer with us, but were costing about £90 pcm)
I kept insurance for our old Labrador as she had a few issues, and cancelled it for one of our dogs because he'd never had anything wrong with him .... I then took out insurance with Pet Plan for our newest and youngest dog (vet said at least take it for the first year as that's when a number of issues can present).
The dog that was left without any insurance promptly ruptured his cruciate ligament - £4K op. He's done the other one this year - less intrusive op costing £1.5k - but we don't think it's worked, so may require further surgery.
Oldest dog, for which we continued paying about £140 pcm, totted up over £6k on ear operations over 2 years and a further £3k on tests in her final weeks. Direct Line even insisted on refunding the £500 purchase price (we got her as a puppy) - even though I pointed out in several emails that this was a bit unnecessary (and not in line with the terms of my policy) as she was almost 13.
Although I used to be in the "put some money away" camp, I think on balance it's good not to have to give cost a second thought when your beloved pet is ill and vets are putting forward options which run into thousands. Even if you've got the money in the bank, it's still a bit of a heart stopping moment when they give you an estimate for emergency treatment which you have to agree to on the spot.
Just as an aside, if you just want public liability insurance, join the Dogs Trust (£20 pa) and you'll be covered, and have access to an advice line.