If people ate money, that would be true. But the important thing for self-sufficiency is not the monetary balance of trade, but the caloric value of trade. It is typical for smaller countries to export high-value refined agricultural products (wine, cheese, etc.) while importing lower-value bulk commodities (grains, oilcrops, etc.) This can cause a vast disparity between the economic and caloric balance of trade. In the case of the UK, it imports about twice as many calories as it exports, so the disappearance of trade would have a major impact. If food waste were eliminated, starvation could probably be avoided. But a few years of malnutrition, while the domestic agricultural sector reconfigured itself for self-sufficiency, would probably be unavoidable.

I was trying to figure that out too.

Part of the problem is that the figures are apparently percentage of value, which might give misleading figures if a country produces a lot of its high-value requirements: producing all your own asparagus isn't much use in a famine if you still rely on 90% imported wheat.