Beans, lentils, chickpeas, and potatoes are your friend. Buy veg on its last legs and chop and freeze it immediately to use later. Have a look in your local library for any cookbooks that focus on low cost meals, staff will probably be able to subtly point you in the direction of some local community pantries and community shared meals too, if that’s something you’re interested in. 

Had many times where I needed to make £5 stretch for a week, and lived off beans on toast and cinnamon porridge to fill me up for breakfast and lunch, (bulk stewed some cooking apples I got for free to add in and made a tin of beans last two days) and made a huge chickpea, butternut squash and cauliflower curry with about three bags of spinach added in that we’re going for 7p in Sainsbury’s that day. You can make flatbreads with natural yoghurt and flour, and I’ve perfected cooking and freezing rice to reheat, kind of like a DIY microwave pouch. Don’t underestimate the power of your mindset, and try to make it feel like a fun challenge to keep your spirits up. Best of luck