The big pots of Greek honey in Aldi are really good value, and Greek yogurt is higher in protein than normal yogurt. You could add some jam or honey. It's also lovely with berries (which you could get if they were reduced).
Op I think you are doing really well on a limited budget. It can't db easy with 3 children on benefits, and trying to reduce debt at the same time.
Both Aldi and lidl do a deal of the week on fruit and veg, which I think are 69p. Asda does the same, but is a little bit more expensive.
Ds2's favourite cheap meal is pasta, just boiled with broccoli and peas and then grated cheese on top. It's also quick to cook, so saving fuel.
If you have a windowsill you could maybe grow some herbs and tomatoes. Post on Freecycle and freegle and see if anyone nearby has some seedlings to give away.
We were given a tray of cabbage seedlings last year, which was brilliant.
I know that financially, unless you have been in similar financial straits, it is very difficult to empathize as to what it is like trying to manage on a very limited budget.
Some areas have fruit ie apples or blackberries on public land which may help to supplement you diet. I think their is an organisation called Abundance in Sheffield which runs a project like this. It could be worth seeing if their is anything similar near you?