Water.. the first priority
Water is your first priority in any crisis. You can go quite a long time without food, and actually it may do some of us good to do so.
If you read accounts of earthquakes and wars you will often hear that the major cause of death among civilians is waterborne disease. Our household water would rapidly become undrinkable in the event of a dirty bomb or even a general strike.
This is doubly true if you live in a flood zone as I do- floodwater would drain into the public water supply and contaminate it.
I am told that an adult needs two ltr of water a day to remain strong. This is total liquid intake which means that you will need much less if you are eating moisture rich foods such as tinned soups and stews (survival favorites).
The second question is to ask yourself how this water should be stored. Many people will be unable to store water in their homes on a permanent basis because it is bulky. After mush thought I came to the conclusion that the most expensive option would save me money in the long run. This would be to buy pickle and piccalilli at Fortnum and Mason in London (just opposite the Royal Academy of Arts) . This comes in airtight glass kilner jars that may then be used to store water.
I intend to build up a collection of these jars over the next couple of years but only a small number of them will be used for permanent water storage. The remainder may be used to store items such as screws and drill bits that are prone to rust. I will place these under my bath- space that is currently wasted.
The total cost for the two jars will be �15 which is expensive- but simply shopping there is an experience.