Spain - not cheap.
I've got some experience living in Spain, and food is generally more dear, and whilst UK supermarkets have their own brand, and then the cheapest-of-cheap 'white packet' brands, Spain knows not the latter. You'll be very hard pressed to find noodles for under 80 cents.
I did, however, spend a month or two, living (more like clinging on desperately) with about 10 euros a week, which isn't a lot more than a quid a day, but for sure, you can survive.
If you spend the whole 5 quid in one go, I'd recommend lentils, a couple of onions, rice, perhaps some eggs and bread, and maybe a little cheap chorizo - if it stretches.
You can whip up a big pan of lentils with rice and eat that + egg sarnies every day. Not very pleasant by day 4, but at least you won't go hungry. Drinking the tap water was a must as bottled water was unaffordable in my budget. that bad.
I got a lot of sympathy for anyone who has to live like this for an extended period of time.