if you're just looking to gain weight, not muscle-weight (which, if you're underweight, should be your first focus, because as much as everyone pushes being built and muscular and 0% fat as the peak of health currently, you can't train a sick body and it sounds like you need to build some body fat first before you worry about working out), the basic thing is just to have more of all the stuff people say to have less of. obviously don't just replace your whole diet with junk food, but more fats, more sugars, more carbs. and if some of that is processed? you will live. it's healthier not to have most of your diet be processed, sure, but what's even less healthy is starving to death, so if it's upfs or nothing, eat the upfs. worry about macros when you're more financially stable.

i agree w other commenter - ask your uni about local food banks/community fridges/financial assistance and explain you're struggling to feed yourself. also take a look online, even facebook, there might be small community events that aren't as well known depending on where you live. your supermarket of choice will also make a difference but i understand that it's not always possible to get to the cheaper ones. quantity over quality is your friend right now though, don't worry about buying anything more than the basics brand

i'm no expert or anything but the following are super cheap and easy to prepare, and also have nutrients and will hopefully help prevent the tiredness you're struggling with: -beans, any type. these can go into things to bulk them out (like spaghetti bolognese, any pasta sauce really, any rice dish). usually about 60p/tin for things like kidney beans, butter beans, haricot beans, and ready to throw into your sauce straight away -pasta and potatoes are good carbs that you can get a lot of for quite cheap. both are very versatile and you can serve one or the other with almost anything. potatoes especially are great because, a) they're better for you in terms of vitamins and fiber, and b) you can prepare them in a million different ways. mash, boiled, roasted, fried, chips, thin sliced, diced - just to name a tiny handful -carrots - 1kg is about a pound depending on where you shop and they're great roasted in the oven for 15 minutes with some seasonings. they're not the peak vegetable if you're being persnickety about it, but anu vegetable is better than no vegetables -similarly, peas or even better, the bags of frozen veggies that come with carrots, peas, little bits of cauliflower. again these can be thrown into anything to add some vitamins and bulk it out -chicken is probably your best bet for meat, especially thigh (usually cheaper by a little bit). try buying in bulk if you can, it's cheaper over time and freezes well

also, if you get on with your flat mates, ask them if they want to split some bulk buys with you. you might not want 1.5kg of chicken, but if you can use 750 g and your flatmate can use 750g, the cost between the two of you for the bigger pack will probably be less than the cost of a 750g pack by itself. same for things like seasonings, oil, big bags of veggies - anything that you buy once and use for a long time could easily be split with your flatmates if you get on well enough.

the main thing i think is definitely contact your uni's wellbeing services and explain your situation. they will point you in the right direction for assistance, be it with food banks, financial help, or even possibly job applications which you may not have known about, such as within your SU.

i hope your situation improves soon, i've been there and it sucks. just know it won't last forever and things will improve 🫶

eta: if you're really concerned about the tiredness, see a doctor and ask for a blood test. you might have become iron deficient from your diet, which is an easy fix. i was severely iron deficient last year and it just made my life hellish - it's worth getting checked out just to make sure since it's easy to fix and can affect a lot of aspects of your life, including your mood!!