When I was a kid my dad owned a chip shop, until he lost his business to a combination of recession and a bypass killing half his trade. While he retrained and looked for new work we were broke and dirt poor. We lived on beans half the time.
You can however get a very varied diet on a pound a day if you shop right. My parents learned the skill and taught it to me - buy bulk, shop at Aldi, look for all those sales where they're trying to clear out stock. Sell-by and use-by dates on a lot of things can be safely ignored for anything from a week up to a month depending.
Oh, and shop at the cash and carry for staples like rice and salt. It's a cliché but if you make sure to stock up on lentils and oats you can survive (not thrive but certainly survive) on significantly less than a pound a day.