£20 per week for food is fairly easy and is what i do more or less..
£4 - 72 weetabix which is more or less a months worth from Farmfoods
£7 - 12 tins of tuna from Farmfoods is more or less a months worth.
£2 - Warburtons bread from most supermarkets (2 per week) comes to £8 per month.
£1.50 - 8 pints of milk each week from Farmfoods comes to £6 per month
That leaves £55 per month in which all you have to do is basically cook your main meals.
Buy 55 packs of Aldi instant noodles for £10, 3 loafs of bread for toast and you're set for 22 days. £7~ left for anything else (some toppings for your toast, perhaps).
Don't go shopping hungry!
You can freeze things like milk.
Someone mentioned batch cooking. Again, freeze what you don't use. Whilst it may last in the fridge for 1-2 days, it doesn't last (generally) for a week or 2.