I’m sorry about your situation, OP, I hope it can be resolved soon.
Sardines in a tin are very good for you, and very cheap. They taste like strong tuna. You can eat them bone in or fillet it out - the bones are a great source of calcium. Sardines on toast is a favourite with me and DPs. If you have any butter and mustard you can mash that in to the sardines with salt and pepper, but it’s perfectly fine just to serve the sardines whole on toast or a but mashed up by themselves.
I don’t agree with the tinned tuna recommendations unless you have some in already - it’s expensive. Try sardines instead.
If you like eating lambs liver and bacon that’s a great source of iron and so cheap for a whole pack in a supermarket. Ditto lambs kidneys. Good recipes online for devilled kidneys on toast.
Pork mince is cheap - you could buy a pack, freeze half and use the remaining half for meatballs - bulked out with breadcrumbs or onion.
Veg-wise - buy British, much cheaper. A bag of carrots, onions, cabbages, potatoes and frozen peas are all very cheap and make great soup with just a veg stock cube added to the water with salt and pepper.
Check out BBC Great British Budget Menu.
Bean chilli is good, bulked out with lots of rice (cheap for a big bag).
Pasta with a sauce made of a tin of tomatoes, drop of stock made from half a stock cube plus whatever veg you’ve got kicking around in the freezer. Macaroni cheese is cheap to make from scratch if you only use a small amount of strong cheddar.
Egg on toast, beans on toast, baked potatoes and a packet of ham. Lentil bolognaise.
Look for meat and fish in the reduced section - reduced beef stewing steak or diced lamb can be cooked for an hour and a half in either a v low oven or on one low ring of the hob with flour and a stock cube for delicious stew (though I appreciate energy use may not be ideal).
Our grandparents had the right idea - make cheap dumplings out of flour and a bit of marg to bulk up meals, or top dishes with a savoury crumble of breadcrumbs or flour and marg to fill yourselves up.
Homemade pastry is cheap - mix plain flour to equal parts marg and add a drop of water- instant pastry.
Also recommend borrowing a copy of Delia’s Frugal Food from your local library (free).
Hope some of these recommendations are helpful. All the best, OP. The way our Govt allows people to struggle so badly makes me very angry indeed.