There will be shortages of food for a number of reasons - none of which are the fault of the EU, or the European farmers, producers or exporters who will of course want to continue to sell to the UK but which are entirely the consequence of the UK's decision to leave the EU.
The problems with JIT delivery of food have been described several times in the thread. If the capacity of the Channel ports is reduced by 60%-70%, there will inevitably be less food available than at present, as about 50% of the food consumed in the UK is imported, largely from the EU.
Even the food from non-EU countries actually arrives in the UK via Europe. It is shipped in bulk to Rotterdam and Hamburg from where it is put onto lorries and driven to the UK. After Brexit, these too will get held up in the queues attempting to get to the UK.
Regarding domestic food production, there are other problems to contend with and the UK has not been self-sufficient for the last 200 years.
British farm animals need food and medicines - largely imported from the EU. British animals are slaughtered and processed in factories staffed largely by EU citizens - over 90% of vets in UK food factories are from other EU countries, so if they go home, all meat processing stops.
Much of the British agricultural crops are picked and processed by EU citizens. For the last century there has been a reliance on migrant workers to meet the seasonal demand for crop harvesting and processing, so this is nothing new, but as was seen this autumn, a lack of workers meant some crops were not picked.
Much of British food is made with imported ingredients. A frozen pizza topping has cheese from Ireland, tomato paste from Italy, mushrooms from Holland, peppers from Spain and ham from Denmark. A shortage of any of these ingredients halts production of the pizza, as does a shortage of Swedish paper in which to package the pizza, and CO2 from France used to freeze it for transportation.
A similar situation exists for almost all popular foods - key ingredients are imported and a shortage of just one ingredient halts the entire production process.
[And rice is indeed grown in Europe - Europe is 70% self-sufficient in rice, growing over 3 million tons a year.]