What is your weekly budget and for how many?
Stretching decent food further is the way to go.
So cottage pies or spaghetti Bolognese, add lentils to the veg. I make cottage pie with frozen mixed veg and lentils and so a small pack of mince goes a long way.
I bought a reduced chicken in Lidl and cooked it yesterday. Stripped the meat off and put it into three different plastic tubs so the size for each meal is more controlled.
Last night we had enchiladas. I used the big wraps and cut them in half before rolling. Added onion and mixed frozen peppers (they cook once it's in the oven). I had a bit of pasta sauce left and mixed it with the remnants of a fajita slice mix (there's always too much). Bloody lovely and very low cost.
Today I'll make a chicken pie (not diet friendly but I only do pastry on top nit a full case and roll it thin). Pack it out with loads of frozen veg. The kids will eat this better disguised as a pie.
Will make Chinese style rice with the last smaller strips of chicken.
Bigger cuts of meat used across several meals are often more cost efficient, especially if yellow stickered.
You are better to focus more on cheap but decent meals as cereal isn't actually a cheap way to fill up and won't help with weight. Although porridge is good. I do ready brek with powdered milk sometimes as we live rurally and often need to make do without fresh.