My son did Uni at £15 a week - Morrisons.
Sainsburys basics meals are about £0.90 and if you look at the ingredients they're not that bad. My DD stumbled across them when she didn't have enough money for a sandwich on her lunch break.
The cottage pie is:
Ingredients
Potato, Water, British Beef (15%), Onion, Cows' Milk, Double Cream (Cows' Milk), Fortified British Wheat Flour (Wheat Flour, Calcium Carbonate, Iron, Niacin, Thiamin), Cornflour, Tomato Purée, Salt, Worcester Sauce (Water, Spirit Vinegar, Sugar, Tamarind Extract, Onion, Garlic, Ginger, Lemon Juice from Concentrate, Clove, Chilli), Malted Barley Extract, Black Pepper, White Pepper.
Would be substantial with a pile of frozen mixed veg (90p kg)
Not saying it's ideal everyday, but the options are there. Also, think of cooking a batch of mince with a basic tomato & onion sauce, portion it out & add different flavours for chill or spag bol or cottage pie.
Veggies from the greengrocer are cheaper than the supermarket I find.
Supermarkets mark down products at certain times of the day - ours is 11am & 7pm. You can sometimes get real bargains.
They key is really menu planning. See how you go on £15 & then you might have to rethink your savings.