It depends a lot on how you intend to go about it.
If you can find a place locally that will do small scale poultry slaughter and preparation then I suspect it will be very straightforward as they will have all the relevant licenses. All you will need is a refrigerated vehicle and storage facility and a food handling cert, I think, unless you can persuade the abattoir/butcher to let your customers pick up direct from them.
My understanding of it is that if you want to go DIY then you'll need to get licensed as a slaughter-man and provide suitable facilities on farm for the killing and preparation of the birds. Last time I checked there were 'easier' regulations for small producers (<10,000 birds a year). You required a minimum of three separate areas all capable of being washed/disinfected i.e. whitewashed walls and stainless steel fittings. One for slaughter, one for hanging (which must be temperature controlled) and one for plucking/gutting. Then you just need the storage and food handling bits as well.
We looked into it briefly for our community farm and decided against it at this stage as it we couldn't find somewhere to take small numbers of birds and process them for us and we couldn't afford to meet the rules and regulations required to do it ourselves. If anyone is doing small scale on farm slaughter and preparation legally in England then I'd love to hear from them!