The main reason the Millenium Bug was, to all intents and purposes, a non-event was because a lot of people had spent a lot of time over several years making sure that it wouldn't affect us to any large extent. I don't have the same confidence in the efficiency or knowledge of anyone in government with regards to the effect of Brexit on imports
Consequently, I've ordered extra supplies of things like pasta, rice, tomato based products, spices, any long-lasting items that come from abroad. OK, if everything goes smoothly I'm still going to use them, and all perishables such as vegetables, fruit, meat, etc I can adapt to what's available so I don't see the point in buying extra of those.
I never thought I would be in a situation where I would actually consider buying to avert shortages, but I have absolutely no confidence in the ability of this government to know what the hell it is doing.