That's comparing production means today with an inflated population due to uncontrolled migration. (Main confronting factor to pop growth in last few years so can't be avoided as topic).
Comparing blockade numbers to peace time production is a false equivalency.
Am I saying there won't be unrest no, but you just started a scenario by blockading Britain. If you think we don't produce enough food, it's about the only thing we actual do produce here other than empty city promises and patents. The country would eat itself before the means of food production hits the top 5 of our problems if honestly bet.
The country has capacity to engage in intensive farming of several means which we currently do not because of animal rights concerns, lack of manpower etc. In a blockade scenario animal rights go out the window to a starving populace. Food waste is addressed in several means and we start engaging in battery farming and crop yields yes move to calorific content vs serving the Ritz their flav d'jour.
And let's not pretend NI is included under any sensible scenario other than the British military blockading and subsuming control of RoI. There's no scenario where a blockade of Britain as an island would leave us "struggling to feed NI with RoI being supported by the European block" as another absolutist.