I haven't read the whole thread so sorry for any duplicate advice.
You can eat way more of most veg than you'd think. Cauliflower leaves and stalk both roast really nicely, same with brocoli stalk. You can add them to curries or shop them finely and stir fry them.
Save veg peelings in a bag in the freezer to make stock.
Use limp past its best lettuce instead of spinach
Repurpose leftovers - fish can be mashed up for fishcakes and frozen, rice can be made into arancini (as long as you refrigerate it within an hour and reuse it within a day), left over veg soup makes a good sauce alternative for pasta, shepherd's pie etc.
If you're running low on sandwich fillings and you have a blender (even a hand blender) you can make hummus easily from chickpeas, lemon juice, oil, and smooth peanut butter (which is much cheaper and easier to find than tahini). I use about a tablespoon of PB for each can of chickpeas and it makes loads. If you can't get chickpeas then it works fine with other beans as well. Of course by the point that you're eating peanut butter and bean paste you probably can't call it hummus anymore, but it stretches a long way and is nutritious and tasty.
If you have any space, lots of veg can be easily grown from scraps. You can grow potatoes from peelings, carrots and lettuce from their tops, dry and plant the seeds of tomatoes and peppers, and lots more.
If you're out of milk you can make oat milk by soaking porridge oats overnight.
You can thicken and increase the nutrition of almost anything by blending a spoonful of silken tofu in with it. Trust me, there is zero increased demand right now on silken tofu haha. I put it in porridge, in sauces, in smoothies - it's basically tasteless and just adds protein.
If you can get to Holland and Barrett you can buy 1kg bags of vital wheat gluten for a few quid, and turn it into a meat replacement called seitan. The only other ingredients you need are something acidic (lemon, vinegar), soy sauce/stock, and water. It's really easy and there's loads of YouTube videos on how to make it. It's about 80% protein so really filling, you can freeze it, and 1 bag of flour makes masses. You can use it to make "chicken" pieces, burgers, "meat" balls, put it in a curry etc.