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The taste of the water in some parts of London or even in some buildings might be unpleasant. Some folks might find comfort in knowing they won’t suddenly be left without water (eg old folks with limited mobility, rather than preppers). There also seems to be various cultural elements related to it: snobbery, distrust of water companies, beliefs about cleanliness.
It’s a real shame that so much plastic is being used for disposable bottles in a city that has potable water everywhere.
Food is fine, assuming you have the cash to fill your cupboards. Water is awkward as who buys bottled water for use in the home? Canned water lasts a lot longer but isn't widely available in the same way.