You might already but try frozen prawns, still not cheap but much cheaper than fresh. Frozen berries are much cheaper, don’t go off and are great for stirring into porridge, yogurt and for smoothies. Instead of flavoured yoghurts I use a tub of Greek yoghurt and either stir in frozen fruit and a bit of honey or if I have fruit on the turn I make a compote type thing (fruit cooked down with a splash of water and sugar).
I make a big pot of chill or bolognese with 750g of mince padded out with tinned lentils. Freeze into meal sized portions and use with pasta/baked potato. A treat meal in our house is nachos made with own brand tortilla chips, chilli and grated cheese melted under the grill.
Bake a few potatoes at once and freeze so if you’re making pasta, you have a potato to pull out into the air fryer or microwave. Cheaper to do a few things in the oven while it’s on.
Im not typical mumsnet in that a 2kg chicken will give us one roast dinner and left overs for a fried rice/pasta/salad for 4 of us. It certainly won’t do 3 decent meals. I find chicken thighs go further because everyone in my house prefers dark meat to chicken breast and they’re cheaper than a good chicken.
The idea of baking break, cakes, flap jack is all well and good but baking ingredients aren’t cheap and it can be time consuming and wasteful if things aren’t eaten. I do bake cookies because they’re quick to make, they’re 10 minutes in the oven and easy to jazz up with chocolate chips, raisins etc and always eaten.
You don’t seem to get much from your £170/week in that your meals are pretty standard, so have an honest look at your recipes, I know things just creep into my trolley if I don’t watch it.