I'm so sorry London isn't feeling better. I've been through the prescription diet thing (cat had severe allergies. Got the food from chewy.com for a lot less than from the vet) and with a very old cat (21 years!) the anything goes experience. With her I had medication that went in her ear (metoclopromide, not sure of spelling) and she was allowed to eat whatever she could tolerate. She couldn't have grains at all or she threw up.

I also had a cat with recurrent pancreatitis and he also got the ear gel med and a low fat diet (I managed without a prescription diet because the old cat couldn't have eaten his prescription so I found foods that met both their needs with a bit of dedicated shopping at the pet store.

At this point I finally have only one cat and she is healthy. It's the first time I've had a healthy cat in many years. She's only 4 so I hope she stays healthy for a long time.

I hope London feels better soon and so do you. I know it is so hard trying to figure out how to make them feel better.