Sorry about taking so long to reply. Things at AADF HQ are really hectic right now. Do you ever find that the more work you do, the more there seems yet to be done?
Anyway, I'm not sure that I can really add too much here as you've pretty much covered it all. GA (previously Golden Acres) is, as far as I know, the biggest pet food extruding plant in Europe. Since very few pet food producers can afford the tens of millions that setting up and running their own factories would cost, most turn to companies like GA for their manufacturing and packaging needs.
GA make foods for all sorts of companies including Burns, Arden Grange, Pets at Home, James Wellbeloved, Natural Dog Food, Barking Heads, Symply, Eden, Simpsons and many many more. They are also the UK's biggest producer of 'white label' foods.
By far their most popular white label products come from the Platinum range: http://www.goldenacres.co.uk/sites/default/files/uploads/GA%20B2B%202013.pdf
Because GA makes such large volumes of these foods, it can do so much more cost effectively than making lots and lots of bespoke batches and by cutting out the middle man (the pet food companies), pet shops can make a much better margin and customers can often get relatively good food at a very good price.
The minimum orders are pretty low and there are no maximum orders so white labels can work for all sorts of companies from small pet shops to large scale online retailers.
Some well known pet food producers like Pooch & Co, Pawtions, Milburns and Europa even pick their products from the white label ranges so they really are everywhere.