Tell that to the people living in the highlands and islands when the Co-op got hit earlier in the year. The nearest other supermarket is 100 miles away, and all of a sudden, it's no longer funny. I was in one a month after the attack, and the place was stripped. The only frozen goods were some ice cream and ice cubes. The only breakfast cereal was high-end muesli. Hell even the alcohol shelves were almost empty.
Now imagine that instead of M&S and the Co-op, it had been Tesco and Sainsbury's, now ~45% of your food supply is gone. Losing just Tesco at 27-28% market share would lead to significant shortages in the shops for weeks.
To my British ears, most of my compatriots who make no provision whatsoever for any disruption in their grocery shopping are as mad as hatters. You don't need a cellar full of tin goods to last months, but if you couldn't go a week without going to the shops at any given moment, then you are a fool of a Took.