I don't know why 'poverty' is cited as a reason for poor food choices.eg; Crisps multipack x 2 = £2.50 Giant choc buttons = £2.52 Hobnobs = £1.89 KitKat 9pk = £1.00 Coke 2ltr x 2 = £3.18 Total = £12.68 That could be used to make 2 days meals for a family of 4 and have some cash over.
That's presuming you have access to a shop (e.g., supermarket) that sells lower priced food, that you have the means and the fuel to store and cook the food, and that you have the knowledge of how to plan out two days of meals for four people on a budget of around £12.
The reality is that a lot of people in poverty are restricted to local shops, usually a corner shop or mini-mart type shop, where the range is limited and prices are higher. If the nearest big name supermarket is a bus ride away then those budget ranges may as well be on the moon - my nearest Aldi is a £6.90 return fare away plus £3.45 return for any accompanying children, that's a big chunk of shopping budget gone before even reaching the aisles. Shopping online requires Internet access, a bank account with debit card feature or a PayPal account, and a minimum spend of around £40 (plus delivery fee).
Corner shops and mini-marts don't usually have discount ranges and the price point is higher, apples that might cost 69p for a bag of five at Aldi could easily be 69p each at a corner shop, if there are even any apples to be bought in the first place. A pack of 10 basic Always sanitary towels that can be bought for £1 in Asda are £3 in my local corner shop. Pasta is £1.50 a bag. Chopped tomatoes are £1 a tin. Cheese is £3 for a small block. Milk is £2.50 for a four pint carton. Bread is £2 a loaf. It all adds up when that's your only shopping option however they sell no-brand frozen pizzas in plain plastic packing at £1 for two and a bag of oven chips is £1 so I could buy pasta, tomatoes, and cheese for £5.50 or I could get two lots of pizza and chips for £2. If I've got £6 to last the week, I know which I'd buy.
Then there's the fuel issue. Food poverty and fuel poverty often go hand in hand. Many people on lower incomes, or without access to a bank account, or with poor credit, or in rented accommodation (or all of the above) are on key meters which cost more to run and mean you are on a finite supply of gas and electricity. If you've only got a little bit of credit left on the meter, you'll cook whatever uses the least amount of fuel. This is why food banks often ask for heat-to-eat type foods such as packet rice, tinned beans, etc.