Think this through. If there is a hard Brexit, the only hinderance to goods coming INTO the UK is the UK. Do you really think the UK will stop or hinder food imports to such an extent that there are "empty shelves" and "riots"?
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.
Lots of faces cut of their own noses.
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.
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.