I've got well stocked cupboards, not because of Brexit/initial lockdown/ second wave, but because I have the space, and because I'm not keen on shopping. I generally go shopping every 10 days, and have probably got 3 weeks worth of food at home.
Preppers did not cause the shortages in March, because they had gradually adding an extra tin, extra pack of pasta/loo roll/ paracetamol to their shopping for many, many months. Preppers don't waste anything because they rotate what they have, it's a way of life, not a knee jerk reaction.
Stockpilers/panic buyers, on the other hand, who rushed out and bought 10 bags of pasta, 10 packs of loo roll and 10 tins of beans did contribute to the shortages, the supply chains were overwhelmed. Most warehouses operate on a "just in time" basis, there aren't massive sheds full of food dotted around the country; stuff comes in, and goes back out.
It makes perfect sense to me to have well-stocked cupboards, and the less often I need to go shopping the better!