As your baby is a newborn you must have muslins around. These can be used as nappies. They won't be waterproof, but they will at least stop the wee and poo going everywhere. If you have a fleece blanket you could cut it up and use it as a cover, that would make it somewhat waterproof. Have a look on YouTube for videos by the nappy lady.
I would call your GP because your baby needs formula a.s.a.p. They might be able to help or even give you a prescription for it, which would make it free.
Sikh temples offer free meals, it might be worth checking if there are any locally. Near us, a lot of churches that are food banks have notices up about what to do if you're in need of food.
Is your older child at nursery or school? Even if they're not attending due to COVID-19, most nurseries and schools have been helping their families in need out. They would not let your kids go hungry.
Good luck, you are not a bad parent. Everyone has been hit with unexpected bills and bank charges, it's just horrible it's come at a time where you cannot absorb it. (Forget what you may have read on here about people with X number of months pay in the bank and blabla, your situation is far from uncommon).
If you let us know roughly where you are I am sure people will be happy to provide formula and nappies at least. I would