To see one, look in damp places such as hedgerows, ditches, banks of rivers, lakes and ponds, damp meadows. It is less common in gardens and parks and dryer, open habitats like chalk grassland
The Green-veined White butterfly (Pieris napi) is widespread and common throughout Britain and Ireland. When flying, it is easily confused with
the Small White butterfly, but when it settles, the Green-veined White has distinctive grey-green veins on its wings. It flies over large areas in search of a mate and breeding grounds. The males sometimes gather together in large numbers on mud, known as ‘mud-puddling’. Butterflies get most of their food from nectar but although it is rich in sugar, it lacks some important nutrients that butterflies need for reproduction and for those, some butterflies visit puddles!
For more info see https://butterfly-conservation.org/butterflies/green-veined-white