Drought

[Mortblanc]

The condition you describe has historically been known as a famine. It was the reason the human race migrated to cover the world as hunter/gatherers, and the primary reason for population reductions in sedentary areas right up to the 20th Century.

The transition to root crops such as potatoes and turnips in central Europe was due to their resistance to being ruined as armies trampled the fields. Some would always survive and hopefully provide enough food to sustain the population. Hansel and Gretal told me so.

A "bad year" could reduce the population of a major nation by 20% or more as happened in China during the '60s-'70s.

It also occurs when a population outgrows its natural food supply, even without the drought situation.

That means that the UK is capable of going into an instant famine mode as soon as shipments of food coming into the country stop, as they did during WW2.

That also makes you susceptible to drought and adverse situations in the supplying countries half way around the world.