To really grasp the potential for disruption, you can't think about one person's Weetabix and a few pallets of extra canned goods. It's the immense scale of provision that's the issue. There are 66 million people in the UK, and the overwhelming majority are wholly dependent on purchased food (ie they don't have a smallholding or hunting rights or a convenient forest full of foragable food).
If even a relatively small amount of food become unavailable, eg no Weetabix or fresh tomatoes, then people will switch to an alternative: cornflakes, tinned tomatoes, whatever. No big deal, here and there. Over 66 million, though, this is a huge switch in purchasing patterns.
If supply routes are still problematic, the alternative food options then begin run out. Yes, businesses have been stockpiling, but there's only so much they can store, and the lack of leadership or clarity on what kind of Brexit we're heading towards won't have helped.
A social effect now comes into play: empty shelf anxiety. Nobody's hungry, let alone starving, but they haven't been able to get what they planned to buy and it's not just the Weetabix shelf that's empty. People are now buying alternative products, but in greater quantities, because it might not be around for long. Why buy one bag of porridge when you could buy two? Or three, just in case? Or - how much porridge can I carry?
This is when panic-buying starts. A bit of long-life stockpiling is one thing; massed public anxiety and empty shelves are another. Even if there's still a perfectly adequate stock of food in the shops (albeit not the food you wanted), this is the point where people begin to buy in excess, and purchases have to be rationed, and the more this happens, the more people worry, and the more they attempt to buy.
Is it a famine, or a zombie apocalypse? No. But there's definitely the realistic potential of unpleasantness and inconvenience, especially if you live in a really high-density urban area. I really hope it won't come to this, but I wouldn't trust our politicians to organise a child's birthday party, let alone a smooth Brexit.
Best case scenario is a lot of food banks will be getting extra donations this summer.