The amount of food that a country exports is orthogonal to its capacity for self-sufficiency. Your example of the Netherlands is a perfect illustration of this. In monetary terms it's a net exporter, but in caloric terms it's a net importer. The latter is a better metric for potential self-sufficiency, and by that measure, the Netherlands is actually the eighth least self-sufficient country in the world[1].
Yes, the UK could undoubtedly reconfigure its agricultural sector to become more self-sufficient. But even on a wartime footing, doing so would take several years. Meanwhile, the slack in food reserves is several weeks. It's that differential between weeks and years where there's the potential for malnutrition and starvation.