My Costo do their own brand dried cat food which is about £18 for 20kg, my lot eat it and seem fine on it. They do tinned felix, 24 tins which is about the same price I think as supermarket and Whiskers pouches. If you know what is on offer where, its best to go prepared as they dont have this sort of thing on offer. However, it is easier to buy in bulk!
I also get frontline there for dogs and cats, again comparable to online prices. Dont know if all branches do this as the staff have to be specially trained and you have to sign for it seperately. Fortunately one of the trained chaps at our branch adopted Squirt (remember him?) and his family from Angie so knows me now!
They do Dentastix for dogs a good price for 72, but as Angie says their range of products is limited.
If you run shelters, then they often have washing powder, fabric softener, kitchen rolls, washing up liquid etc on offer when they are much cheaper.