I’d maybe start with some chicken. Tesco do 2kg of Chicken Thighs for £5.75 or Chicken Drumsticks are on offer at £3.75 for 2kg at the moment. I’d get chickpeas (less than 50p a can) and tinned tomatoes (same price). Some onions, pack of courgettes and a pack of value wonky peppers. A jar of pesto, a couple of cans of baked beans and a block of basic cheese. A couple of tins of Tuna or Sardines or Mackerel (depending on your tastes) and a loaf of bread. This should come in under £16 based on prices in Aldi and Tesco (as of today in my local stores) if you go with the drumsticks and the more basic ranges. Check out the Aldi Super 6 and other offers. UHT milk is cheaper than fresh and would be fine for porridge or overnight oats. You’ve got a bit of leeway for more oats, pasta, bread, or a wee treat.
If you want to save on cooking you could season and roast all the chicken together, but things will be nicer cooked fresh.
I’d do some roasted chicken and roast veggies for one meal, that could be served with pasta and a sauce from the tinned tomatoes.
You could make a chicken and rice dish with the rice, chickpeas, onion, a pepper, a courgette, spices you have to hand and some more of the chicken.
Use some of the cooked chicken for your picnic, either as drumsticks or as a filling for sandwiches. Make a salad of your cucumber and tomatoes and take the cherries.
Make a veggie pesto pasta with another courgette, pepper and an onion. Top with cheese.
Could you bake the large potatoes? Or chop them into wedges/chunks in their skins? Bake in the oven (or airfryer) then top with baked beans and cheese.
Make toasties with the tinned fish, very nice grilled on toast. If you go with Tuna and you have a bit of cheese and onion left you could do a Tuna Melt.
If your budget is because things are tough there might be a food pantry or community larder in your area that you can access without any kind of referral. I used to volunteer in a little shop like that in our community and we also often had pet food available for free.