To kick start your preps, buy something like a 5kilo bag of rice or a couple of 3 kilo bags of pasta: Cost less than a tenner. That's your calorie source for many weeks in one simple go.
Next week augment with maybe half a dozen tins of everyday value tinned tomatoes and/or beans: That turned your pasta or rice into MEALS you could survive on.
Then, just pad it out with stuff to give nice variety: Maybe a kilo of porridge oats and a tub of powdered milk. Keep cost low buy buying acceptable quality 'value range food' Don't give two hoots what your fellow shoppers or checkout operators think of you.
Bingo. You're ahead of the neighbours.
Going forward, treat your stash as your grocery store and your grocery store as your wholesaler. That way, all food you buy gets rotated and you actually find yourself buying what you normally eat.