The clue will be in the queues outside every food shop, followed by food riots.
Unlikely.
The UK produces more food than it consumes. Currently, instead of scoffing it all ourselves we export a little over half of it. Profit before strategic reslience.
If there were a food emergency of that scale we could simply cease exporting it and eat it within our own shores. If that were to happen there would still still he a surplus.
So while I would normally nod sagely at your writings, on this occasion I must respectfully disagree. There simply would be no food riots because, ultimately, in the event that food ceased to cross international boundaries we would still have more than we could eat produced within our own shores.
Realigning domestic distribution networks would be far more problematic than the actual availability of stuff to eat.