It seems simple to me:

because of Brexit, the UK supermarkets decided to switch their custom for some varieties of fresh veg from EU producers to non-EU producers, notably Morocco.

Morocco has experienced bad weather and production has thereby been curtailed.

At the same time, British growers were impacted by fuel costs to heat greenhouses absolutely spiralling out of control, resulting in many of them shutting down their growing operations due to the impossibility of making a profit at the margins that the big supermarket chains allow them (which are, in effect, next to nothing).

The current tory government have shown themselves to be totally useless at ensuring the Britain has food security as they twatter on about market forces being the answer etc etc. Meanwhile in mainland Europe there are no shortages whatsoever of fresh vegetables; the shelves are groaning under the weight of fresh tomatoes!

I live in Luxembourg and I can attest to this. The other day I did a quick shop in my local Aldi where there were 5 varieties of tomato in stock. Yesterday I went to my local Colruyt (a Belgian supermarket chain) where there were substantial quantities of around 10 varieties of tomatoes on the shelves!

Is Brexit to blame for the current empty shelves in the UK - well yes, for the most part, yes it is to blame.

Were präts like Rees-Mogg right about being able to save money on food after Brexit? Yes, of course they were - if there are no fresh veg, you can't really spend money on them, can you?