This is a very widespread butterfly being spotted across the whole of the UK. It is a small white butterfly, while the underside has a green tint to it. The common name for this butterfly is the Green-Veined White (Piers nap).
It can be found in damp environments such as woodlands. I spotted a couple of them white out for a walk in mid May to the Angmering Park Estate. This wooded environment made for the perfect conditions for them.