I personally would boycott any shop or supermarket that sold food that was raised on lower standards than presently applied in the UK and Europe, especially so with chlorinated chicken sheds where chickens are raised very very poorly.
the thing is, you probably will never know. The legislation isn't just about not having standards enshrined in law, it also affects labeling. The Americans think nutritional labeling is bad. In the absence of a law, it could easily be dropped from UK labels as part of a trade deal. The US doesn't label GMO food, and there's also the issue of antibiotics. It's unlikely that these will be flagged on the packaging, as the US wants to “eliminate … unjustified labeling” saying it unfairly discriminates against American foods and, get this, “views the introduction of warning labels as harmful rather than as a step to public health”. It has even recently challenged country of origin labeling.