There are a couple of Bedfordshire poultry farms on this website. Why not give them a ring? Even if they don't have the hens you want they may well know where you can get them. (btw my old boss - in the farming world all his life never called them chickens - always hens or poultry!).
I know there's one in Days Lane, Doddinghurst, Brentwood, Essex. The lady does lot's of different chickens for show, and sells them. hope this was of some help, maybe a bit to far to travel