I'm on a very limited budget too & I shop for bargains all the time. I've built up a pretty big stock & it hasn't broken the bank because it's been gradual.
I get 500g packs of spaghetti from either Tesco or Aldi when I'm in either for 20p. 5 packs is only £1 & there's a lot of satisfaction at the start seeing a pile of something that hasn't caused sleepless nights buying. Rice from either of those shops is good too at 45p for 1kg. 2 bags of rice & all that spaghetti & it starts to look like a respectable amount. There can be slightly cheaper prices if you're buying in much bigger quantity packs but to be honest I rarely would have the £7 or whatever available for just 1 item no matter how large it was.Little & often suits my purse.
I got 3 large tins of Corned Beef in Aldi today for £1.39 each, normally £2 there & £2.50 now in most other places. If they still have any at that price when I'm back there again I'll get some more.
In Tesco's I got 3 x 300g of their own brand coffee granules for £1.46p each. They're reduced to clear & again if they still have any when I'm back tomorrow I'll get some more. I got 6 tins of a savoury mince there a few months ago reduced to 25p each. Keeping your eyes open for big reductions pays dividends.
I pick up one or two tins a week of their spaghetti hoops for 13p & maybe other cheap tinned vegetables like chick peas, chopped tomatoes at around 30p - 35p each, their cheap curry/sweet and sour jars of sauce are only 30p. They've packets of noodles for 14p that I've bought loads of over the weeks.
I don't store water but I've got quite a few collapsible water containers from the Pound shops, I think they hold about 10 - 15 ltrs each if memory serves & if water looks like it's going to be a problem I'll just fill them up straight away from the tap. I've water purification tablets as well.
Candles, again from the Pound shops. Batteries. Matches. I've 2 wind-up lanterns & 3 oil lanterns as well that I bought from ebay about a year ago. The Pound shops tins of meat are good to have, ham, chopped pork, I even got a few tins of pulled pork on a great offer there of 2 for 50p. An extra bottle of shampoo, soap, toilet rolls etc from there every so often wont break the bank. They have paracetamol 3 packs of 16 for £1, good to have on hand. They do 2 x 1ltr cartons of UHT semi-skimmed milk & again I might pick up 2 every couple of weeks or so.
Tesco's cornflakes 750g boxes for 75p are delicious. I prefer them to many of the Aldi cereals.