The UK imports around 25% of food-related commodities from the EU. Even with no deal, these imports will continue (as the EU has to sell them to someone.) Even if the EU stopped, there would not be corpses in the street as there happens to be a very wealthy English-speaking country not too far away that happens to produce more food than any country on the planet and is more than willing to trade with the UK.
https://www.theguardian.com/food/2018/sep/15/food-and-brexit...
So it's not a question of whether the US will trade with the UK, in the long run. It's a question of whether, 2 days after a hard Brexit, everyone has been well-organized enough to keep food flowing across the border at the same rate as always, despite the massive customs upheaval.
The economic concern of a hard Brexit isn't the imports, but the exports. There might be significant economic disruption if the EU slows down trade of items coming INTO the EU and OUT OF the UK. But, the UK is solely in control of how smoothly goods coming INTO the UK and OUT OF the EU get distributed.
Have you worked with bureaucrats? They won't view it as stopping, but rules and paperwork need to be filed. If processes are followed stuff will flow, but there will be a flow stopping shockwave propagating backward at the rate of tps-1c.
This is because, as an EU member, the UK has trade agreements with many countries outside of the EU. After a no deal Brexit, it has none. This affects all UK imports and exports.