Actually Lottie, that comment isn't strictly true, VetUK do sell prescription only items, so not everything on the site can be ordered normally.
Lisa, I would see how well she takes to i/d first, as you normally have to buy large amounts for prescription foods, I dont know how the pouches work, if you can just buy one box, but with tins, you have to buy 24 of them in one go, which isn't good if htey dont like them.