Yep, far better to find the local planning policies and read through all of them and base your objections on the areas that are close to breaking/bending those policies. Planning policy documents are not little things and are interwined with any "Local Plan" and what given areas are defined as and what is "pemitted development" within those areas.
Like increased level of HGV traffic, chicks in, chickens out, feed in, waste out, increased noise and increased pollution from this traffic, damage to property from vibration, suitabilty of the local roads, access to the site during construction and once in operation, odour from the site, visual impact, etc.
The "emotional" chicken welfare stuff will be simply countered by the operators saying the place will be run to the latest standards required. Planning can't impose anything more than that.
It can be done but it'll take over two or three peoples lives for a year or three. Organise a local petition to the Planning Authority, Write to your MP, write to your Parish, District and County coucilors, encourage others to write as well. Try not to use "form letter", 50 individual letters carry more weight than 50 form letters. Perhaps have a "menu" of objections that people can choose from to express in their own words.