The Panorama programme raised a number of points of concern to any UK consumer considering buying US meat: the use of unnatural food fed to fatten up beef cattle,
Easy problem to solve. Market (consumer) preferences can dictate how beef consumed in the UK is raised (including fed) before it reaches the butcher. I won't pretend to know what feeds are allowed in the UK. I assume grass-fed beef is considered OK? Grass-fed is produced in the USA.
Insist on that, and producers will comply, regardless of country of origin. Personally I don't eat beef/veal, lamb, pork, or any mammal meat (for medical reasons), but my family does, they always go for grass fed beef, and other red meat sparingly.
of restraints preventing movement of pigs
In my environmental consulting days I sometimes worked on farms, including farms that produced pigs for slaughter. Pigs were in pens, but had room to move, never restrained. There are bad actors in every field, is the footage from the worst farms?
the use of antibiotics to an extent greater than allowed by the EU
Again, easily dealt with by the UK indicating to producers in any country what will and won't be allowed in the UK market. Most likely verifiable with testing, though I'm no expert. Consumer demands can dictate other additional quality requirements.