Okay, let's break this down.
First things first I would think about breakfast and lunches.
Do you like porridge? A big bag of oats would do a good few meals and they're filling. You can even make non-microwave oats in the microwave just fine. Top with some frozen fruit/nuts/jam depending how the budget is looking after lunches and dinners.
Then let's worry about lunches. I'd probably go down the basic sandwich route here. Do you like e.g. cheese and pickle or cheese and cucumber? Cream cheese and cucumber. Add an apple and/or banana, and you could get done inexpensive biscuits as well if it's feeling like you'll be short on calories. It's not great but I could live with eating similarish things most days for a few weeks. I'd also consider eggy bread as you could have that with a bit of ketchup and some cherry tomatoes/cucumber/basic salad and that would be pretty filling.
Then you're onto dinners.
If you like macaroni cheese, you can make your own of that but it does require a lot of stirring.
Baked risotto is a fairly low input dinner - fry some onion (you can buy it frozen and diced if you'll struggle with that) and then add the risotto rice and some stock. Shove in the oven for 30 minutes and at the same time you can roast some butternut squash (again you can buy it frozen and diced if it will be too difficult to do). Get the pan out the oven, give the rice a really good stir to get the starch going. Add some cheese, cheddar is fine, and serve topped with the butternut squash. You can add some dried herbs etc for interest. (Frozen butternut squash, £2.10, risotto rice £2.40, frozen onion £1.65, cheese 2.69, but again this will be enough for 2 nights and you'll have lots of leftover cheese and onion from this. £9 but with plenty of leftovers,)
Another low input dinner is slow cooker spaghetti meatballs. Buy meatballs (frozen, surprisingly, sometimes have better ingredients and are cheaper). Buy a jar of pasta sauce and a tin of tomatoes. Buy a pepper and a courgette. Ideally fry off the meatballs. Add the pasta sauce and the veggies, a stock cube, some garlic and some herbs. Could shove some frozen onions in as well. Bring to the boil. Shove the lot in the slow cooker all day, ideally with a few cubes of leftover frozen butternut squash/sweet potato as it helps it thicken up. Eat with spaghetti and optionally salad/garlic bread. (Meatballs for at least 4, £4.50, pasta sauce 69p, tinned toms 47p, pepper 59p, courgette 56p, spaghetti 75p, so £7.60 but I think it would do at least 2 nights, probably 3).
So if you do those I think that would do 5 nights for £16.50 ish maybe £18 with some garlic and stock cubes, so you'd have the same again to play with for your last two nights. Then see where you are after you've done the basics for breakfast and you can decide how you'll jazz the food up - be that nuts for breakfast, extra salad at tea or some garlic bread etc, with what's left in budget.
I'll have a ponder for other ideas along those lines. I think you could make enough for 4-6 for each of those without breaking the bank and that would do 5 nights of the week.