This seems to be the most humane and stress free (for bird and farmer) method of dispatching table birds, but I can't see how you could get the clearance as a smallholder (private consumption). WASK mentions presence of vet and a licensed slaughter person, plus notice given to Sec of State five days in advance. It also states that the birds will have come to end of their life as either layers or breeders.
It is a lot cheaper (the kit would cost £50 for the chamber, £20 for a regulator plus the cost of a CO2 or Argon cylinder which is re-fillable) than an electric stunner and would leave less of a margin for error for a well meaning, but incompetent person trying dislocation.
Any thoughts?