There is a business in my area of Texas, Natural Pawz, that is doing this and quite successfully too. They have a website, and I think they're up to 4 stores now. It is owned by a Canadian couple who got into it after adopting a dog with a lot of allergies.
The blog would be a good idea. Where I am in Texas, I'm established as one of the "go-to" people in my area when it comes to small-animal care. As a matter of fact, a dog/cat rescuer e-mailed me last night, asking for tips on clipping guinea-pig nails, because someone had asked the question on Facebook. I've also had to share a "recipe" for the pellet slurry used in syringe feeding, because Science Recovery in the UK is available only via vets.
Non-GMO food is a big thing, so I think you could do this, with the proper introduction in the right markets. For cats and dogs, grain-free seems to be catching on. I have a friend who has to keep her rescued dog on a grain-free diet, and I've sometimes switched my cats to grain-free, when recalls of popular brands were widespread (the melamine scare).
To go with your organic theme, you might also keep a list of vets that practice integrated medicine on your website. Chiropractors who help animals would also be good on that list too, if this is legal in the UK.
Good luck!