Many white butterflies with dark veined undersides.Probably Green-veined whites? Not good with whites
No interactions present.
Nice to see these butterflies grouped together, and if you click through to the original full-size version of Graeme's excellent photos you can see that most of them have their proboscis out, 'feeding' on the moisture and salts in the wet mud.
This behavious is sometimes known as 'mud-puddling'; for an American perspective on why it happens see:
Entomologist and biological recorder
Lat/Lng: 51.2, -1.5
OS grid ref: SU3546