It's perfectly possible for a single person even at Tesco:
Monday:
Breakfast: Banana porridge
Lunch: Tuna nicoise salad
Dinner: Veg fajitas
Tuesday:
Breakfast: Banana and yoghurt
Lunch: Leftover fajitas
Dinner: Chicken drumsticks with couscous and salad
Wednesday:
Breakfast: Banana porridge
Lunch: Leftover chicken and salad
Dinner: Brocolli pasta bake
Thursday:
Breakfast: Banana and yoghurt
Lunch: Leftover pasta
Dinner: Brocolli, mushroom and pepper stir fry
Friday:
Breakfast: Banana porridge
Lunch: Leftover stir fry
Dinner: Falafel wrap with salad
Saturday:
Breakfast: Banana and yoghurt
Lunch: Lefover falafel
Dinner: Pepper and brocolli omlette
Sunday:
Brunch: Scrambled egg and mushroom tortilla
Dinner: Roast chicken drumsticks, brocolli roast potatoes
Pack of peppers - £1
Mushrooms - £1
Onions - 70p
Pack of 6 chicken drumsticks - £3
Brocolli - 50p
Tin of chickpeas - 40p
Box of 6 eggs - 90p
Couscous sachet - 50p
Pack/pot of yoghurt - £1
Bunch of bananas - £1
Tortilla wraps - £1
Porridge oats - £1
Milk - £1
Romaine lettuce x 2 - £1
Cherry tomatoes - 90p
Bag of pasta - £1
Pack of noodles - £1
Potato - 40p
Tin of tuna - £1
Mild cheddar - £1.50
= £19.80
Obviously that's not the most thrilling meal plan in the world, and doesn't include toiletries or cleaning products, but next week you'd still have noodles, pasta and oats so instead of buying those you could get Radox 2-in-1 shower gel and shampoo for £1, a bottle of antibac cleaner for £1 or some deodorant or loo roll.