Re: Why would they?
God help us in the UK if Brexit goes badly and we have to start importing more food from the US, as we'd have to weaken our food safety laws to let the stuff in.
We already import lots of food from the US. Florida orange juice and California wine, to name but two I generally avoid in favour of alternatives from elsewhere.
The issue isn't how much we import, but whether we have a general trade agreement. If we do that, we'll have to accept US "red lines", that prevented proposed trade agreements with either the EU or Pacific Rim countries under Obama and Bush - back when the US at least believed in trade. That'll mean not just weakening food standards, but also any labelling (like "red tractor") that could be used as a proxy to discriminate against products like US growth-hormone-filled beef.
BTW, the scale of smuggling likely to follow that is why a trade agreement with the US must imply border controls for Ireland.