Marbled White (Melanargia galathea)
Interestingly it is not a 'white' it is a 'brown' despite having no brown on it. It actually belongs to the family Satyridae of which most of our brown butterflies also belong hence the colloquial term 'browns'.
The Marbled White is a butterfly of the grass downlands, especially on chalk or limestone so it is quite common here in Dorset. It does also crop up in other places where there is long grass as its food plant is Sheep's Fescue, although it has been raised on many other species of grass in captivity.
Single brooded you will find it flying now until the middle of August.