Wheat or the more expensive mixed grain can be given. The hens will also eat the growing tips of grasses, and anything they can forage from their scratching activities. Also provide some poultry grit to help grain digestion in the gizzard, and crushed oystershell for strong eggshells. There are feed suppliers who specialize in supplying good quality free-range feeds based on natural ingredients from plant sources. They advertise regularly in Country Smallholding magazine and their products are available via local suppliers.
Organic feeds are the only ones that are guaranteed to be free of genetically modified ingredients.
The term 'natural feed' has no legal meaning. Chickens have a small digestive system and need to eat small amounts frequently. It's much safer, and more convenient, to give them a ready-made mix. Yes, as long as they are fresh and contain no meat or excess salt, and that you are not selling the eggs. Don't give too much or they will not be getting a balanced diet.