Farmers tend to cry "food security" pretty readily.
A threat to British food security would need to come from:
-
Blockade. There are few countries that are able to blockade the UK, and in most of the cases, I suspect that there would be more-immediate problems for the UK.
-
A scarcity of food on global markets. Given that the UK can afford to outbid most countries in the world for food, if the UK gets in trouble, most of the world is already screwed.
But, hell, let's ignore all that and for the sake of discussion, assume that there's a real food security issue. If so, then:
A. The UK should stop producing foods other than staple foods, stuff like wheat, since this is the most calorie-productive. No meat production should occur, as this is quite calorie-inefficient. I strongly suspect that the National Farmers Union will suddenly decide do an about-face on food security being an issue if this point is raised.
B. The UK should be worried about its supply chain for food production, not simply food production. For example, "The UK is estimated to produce approximately 40% of its nitrogen fertiliser requirement, while the remaining 60% is relied upon from imports (AIC)." That's a higher percentage external dependency than is present food production.