I hear what you are saying about getting one shop so you don't buy extra stuff as you shop. But I do the opposite when I have a limited budget.
I divide what I have for each day and take it out or move it from my bill account to my ordinary account. Or have it all in cash and put it away in an envelope and only take out your day's amount each day. It makes me feel safer that I don't blow the whole lot now and have no food or money left for the last 2 or 3 days.
So £80 divided by 14 days is £5.70.
You can do fiver a day with some extra one day at the weekend.
Look at what is in your cupboard. Can you make a meal today? Even if it is a mish mash of things. One person having beans on toast, one having a small omelette with the last two eggs, that sort of thing. Or a packet of rice and any leftover veg can make a risotto. Last bit of pasta left in a bag and one of the kids happy with it on its own (my kids have always liked that).
Then that means you have double tomorrow. Go to the shop and buy some pasta and some tins of tomatoes. That's one meal. A bag of cheap onions always helps for flavour. Buy a bag of potatoes (not 4 bakers - a full bag, usually about the same price these days if you get the basic bag). Feel the bag to make sure there are a few bigger ones that you can bake. The rest can be used another day in another way. Value beans. That's another meal.
Porridge or non branded weetabix or cornflakes. Milk.
Red lentils, carrots and cheap stock cubes to make soup, that will do lunches for a few days. Cheap loaf of bread to have with soup or for breakfast or as free for alls when someone is hungry.
That should last you a couple of days. Then you not only have some things leftover (lentils, carrots, stock cubes, potatoes, breakfast stuff) but you have a new day of cash to buy the next day or two.
A bag of white rice and some green lentils, kidney beans, tin tomato and you have a chilli one night (if you have any spices). Green lentils can make a nice shepherd's pie with the potatoes. Add in some of the red ones too to both dishes.
And if you have money left some days either roll them over to the next day or buy a value pack of biscuits as a treat. Don't worry about the over all health for two weeks. Just fill up with as much as you can. Go round the shop with your fiver or tenner if it's two days and look whats cheap and on offer. Tesco have a frozen cheesecake for instance for less than a pound (used to be 65p i think over 80p now). That is my son's go to treat when we are on a "how much can we get for our budget" shop. That can be a treat at the weekend.
I know this isn't everyone's way of doing it but it makes me feel safer for some reason.
It isn't a huge amount OP and I'm sorry that you are in this position. Good luck.