If you're on a very tight budget then, if you are able, do the rounds of the supermarkets in the evening, between about 6 and 8 when they are reducing items down to 20p or less. I rarely pay more than 10p for bread and the freezer is full of it!
Also, at this time of year look out for apples and sweet chestnuts if you can find some trees near you.
Basics would be porridge, rice, pasta, potatoes, lentils (red ones cook quickly), tinned tomatoes, baked beans, cooking oil, plain flour, onions, margarine, milk of some description (cows milk is often reduced to pennies at the end of the day). A cabbage will go a long way, either as a vegetable or in a stir fry. Carrots and swede are cheap and are good for soup. Eggs will make a meal, scrambled go further. Sardines are cheap if you are non-vege. Fruit is nearly always reduced at the end of the day, as is salad stuff. Our nearest Asda bags up all the loose and slightly bruised bananas and a big bag works out to be about 20p. Good for banana cake if they are too squashed.
I have bargain shopped for food for many, many years. It has now become a bit of a hobby!