Defra has information about this on their website and you can find chicken transporters by googling............
When I bought some chickens (Derbyshire Redcaps) from the rarebreeds society about 10 years ago they had to be transported by a licenced carrier (it cost a bomb!) who would stop, feed and water the chickens as defined by Gvt regulations. They had to be carried in special 'compartments'.........
If you are doing it yourself, it is a different matter and commonsense, as the welfare of your chucks will be your prime concern.