In terms of the milk - use Whole milk, and also whilst in most supermarkets 2 pints is £1.25, 4pints is only 30/40p more - so 4 pints for £1.55/£1.65
That's a lot more nutrition for not much more money. You can also freeze milk (in ice cube trays if you want the right amount for a cup of tea) - will need to freeze in tray them then decant into a bag, sealed tight for best results.
Pasta/curry sauces from tesco - their Hearty meals range are about 45p each - cheaper than making your own (not as good nutritionally though) - you can start with them and then add more passata/ tomato puree to expand them a bit.
Potatoes - baked, mashed, wedges, chips, spanish omelette, potato bread, boiled, roast, patatas bravas - or just add beans, cheese, bacon scraps, curry sauce, tom sauce, whatever you have to make meal.
You can buy cheap rice pudding for 25p at Tesco - add a dollop of 40p stockwell jam and it will fill you up.
Porridge 1kg 70p , Cornflakes 76p - use with the extra milk above - for lunch, breakfast or just when you're hungry.
Get some cheap biscuits - just for morale.