It could go much much worse than just "bad".
The UK imports a lot of food from EU neighbors. In case of a hard Brexit, this will just ... stop. The question is whether all supermarkets can immediately redo their supply chains. That is not likely. Especially since the UK will loose all trade agreements with the non-EU countries as well. Because they had been made by the EU with the UK participating.
Our western societies are very efficient but also very fragile. If supermarkets can no longer fill up their shelves, people go hungry fast. If supermarkets can only fill 80% of shelves, people panic and start hoarding food. Those who are too late to the party go hungry. With enough people hungry for a few days, society collapses.
Like ... corpses in the streets.
We know what happened when the Soviet Union went down and shelves went empty. This scenario would end a lot worse for the UK. The Soviets had massive redundancies, inefficiencies and fallbacks that could be exploited. Such as people growing potatoes in their Dachas.
See the material by Dmitry Orlov on how the "soviet scenario" would play out in a western country:
https://www.resilience.org/stories/2006-12-04/closing-collap...
http://cluborlov.blogspot.com/2009/02/social-collapse-best-p...
Update:
What do you think "chaos at the border" means? Everybody will try to get as much stuff over the border as possible. Everybody will work together. The EU, the UK, the US, everybody. With the best intentions. But this could still go very very bad.
Every truck, train and ship has to get the papers checked where they could just drive through before. But there are not enough qualified officers to actually check all the papers.
So goods will come in at a trickle. Even with the best intentions.
This the UK governments own statement. Chaos at the border for six months:
> Britain’s government has revised its worst-case Brexit scenario estimates for chaos at the country’s borders, which it now expects to last for six months rather than six weeks, the political editor of The Sun newspaper said.
https://www.reuters.com/article/us-britain-eu-borders/uk-now...