It was developed by a rat breeder and popularised throughout the NFRS breeder community. Her ebooks are here: Rat diet EBooks - The Scuttling Gourmet (shunamiterats.co.uk). I haven't read them but I think they explain the theory behind it.
In terms of rat foods available to buy in the UK that are worth feeding, it's either Science Selective pellets or a Shunamite diet from Rat Rations and a varied mix is of course always recommended over pellets.
The rabbit food used is supposed to be the muesli kind, not the hay based pellet kind, but it can also be substituted for a mix of minimally processed grains which I would think would be better. Likewise, the dog food can be substituted for insects and freeze dried meats which I would think would also be better. I think much the the development of the mix is based around the owner's and breeder's convenience. Likewise with the human cereals, they're easy and cheap to obtain. I would also think that rats having adapted to live amongst humans, would naturally be eating more grains, both processed and minimally processed, than would hamsters, for example.
I will add that while I really enjoy the UK online rat community, one thing I have noticed is that on certain issues, one opinion will be pushed to the exclusion of any other. It's not that comments are ever deleted or other views censored, it's just that the community is very united on certain opinions and nobody really bothers to check out other opinions. So on a question on diet, nobody will suggest anything other than the Shunamite diet and it's hard to know if anyone really does anything different.
Edited by daisy03, 11 January 2021 - 10:37 AM.