First, I will say that I've had a polite warning from moderators about a thread I started that wound up verging into political issues, and a post from me that weighed in on those issues as expressed by others. This thread IMO includes some posts that verge into anti-USA bias and also against other nations. I'm asking only that rules be applied fairly.
I will weigh in on what I know of US poultry production (in a former job, I worked on environmentally-sound waste management for such companies, among other work). All poultry produced or sold in the USA MUST be hormone-free, by US law, and must bear markings on the packaging indicating it is hormone-free, no exceptions. Including antibiotics in US poultry feed used to be common, but many producers realized consumers don't want the antibiotics, and have eliminated them from poultry production (to be honest, not all producers have made this change). I don't know what other specific chemicals might be a concern(?). To meet customer demands, US growers have adapted to produce what customers want, if UK buyers have certain requirements, the producers will adapt.
I have seen lurid footage in anti-meat production films, and have also been on such feedlots for poultry, pork, and beef production. There are bad actors in all industries, so take those films with a big grain of salt. The feedlots are not ideal, but are not as bad as the films. There is also a lot of grass-fed beef production in the USA, if UK consumers want that, convince the UK markets to buy those products, rather than corn-fed beef, fattened on feedlots.
I eat US poultry, not US beef or pork, due to health reasons. When my wife buys beef for herself and our sons, she buys free-range grass-fed beef. We have substantially reduced our overall meat consumption, because eating less meat is better for the environment.