“France is the European Union’s largest grain grower and exporter.”
https://www.france24.com/en/france/20211014-rising-fertiliser-prices-may-prompt-french-farmers-to-switch-grain-crops
And there you have it, almost nothing to do with Brexit. Almost everything to do with World wide demand for Hydrocarbon fuels. If France, the largest agricultural producer in the E.U. is concerned by lack of fuel and fertilizers, then it is more pandemic than Brexit.