It is some years ago that I first became interested in the composition of pet food and the website that helped me most at that time was The Campaign for Real Pet Food. AFAIK it was set up by a number of companies with the intention of helping owners to understand what they were feeding their pets. They particularly campaigned for clear labelling of pet food. I can remember that Collards, Feelwell’s and Naturediet were involved but there were more.
I found it useful, particularly as it also had a forum. Unfortunately it was not well used from what I remember. The website is now down and the Facebook page has not been updated since 2013. Does anyone know what happened to it?