Hi Sarah,
I am lucky enough to know a kind organic butcher that gives me a huge bag of chicken carcasses, lamb ribs, and pork bones every week - so my dogs are mainly fed on organic raw meaty bones. However the tripe I get is from AMP so that isn't organic.
I think organic is ideal -
same as it is with us humans - but if you can't get organic, then the pet meats you can buy frozen are the next option. They may well have "chemicals" in them if the animals have not been organically raised, but at least they don't have colourings, preservatives, and other such additives in them.
Have you asked on Britbarf if anyone knows of an organic supplier near to you ?