UK has relatively high livestock and animal welfare basic regulation, and in addition it has a certification scheme (that works out about 25% more expensive), run by the "Royal Society for Protection of Cruelty towards Animals". They base their standards as much as possible on academic measurements of stress and natural behaviors. One thing to note is that animals tend to get ill the more 'unhappy' their existence is - and this is argued as one of the reasons there is no need to chlorinate chickens from UK and very little food poisoning risk from fresh produce. Anyway, the RSPCA will actually certify chickens that are not able to 'free range' (these days it means ability to go outside occasionally) Their advice is it doesn't seem to be of great importance to chickens, unlike stocking density, temperature and conditions inside the barn, and even the introduction of 'toys' like beachballs which some are observed to play with. RSPCA workers are commonly regarded as genuine and professionally experienced their charge of, preventing cruelty towards animals.