I am so sorry you're going through this, my family was very poor growing up, I remember mums of friends crying because things were getting more expensive on a daily basis and we pretty much lived on potatoes and toast, though we did have an excellent local undercover market where food was local and very cheap so we could get good fruit and meat. Many of the stalls have now been replaced by "artisan" providers...We also had the Greggs seconds shop, which was food that was a day or so old (I guess Too Good to Go has taken over that role in some places).
I can see people have already mentioned Too Good to Go and Olio; for occasional treats, is it worth looking at some of the Apps (Greggs for instance) as sometimes they have special offers, especially when you first sign up.
And B&M, Poundland, Home Bargains also sometimes have food including treats at lower prices than Tescos, Morrisons, etc. (It did make me laugh that Rishi chose Sainsburys for his photo ops - I can't remember the last time we could afford to shop there!) We also have a community fridge in our town - just opened - would it be worth seeing if there's one of yours nearby? The stuff in ours is totally free.
I'm totally with you bulking things out as well - lentils, oatmeal, soya mince, breadcrumbs, milk to make chicken soup go further, grating the cheese and freezing it, blanching onions and other veg and freezing them if we have the space...
I never thought so many of us would be facing this again in my lifetime. It's appalling, and the gulf between rich and poor is getting wider every day.
So no, you really weren't being unreasonable. I just wish you didn't have to go through this.