Ok, here is my shot - not too difficult this one.
Which common and sexually dimorphic butterfly was once considered as two species and named The Turnip and The Colewort?
Jerry
Sorry - wrong.
I am surprised it has gone so long.
Perhaps the sexual dimorphism bit has fooled you.
It was not the sexual dimorphism that led to the thought of two species, but variation.
Perhaps that helps.
Guy
Correct Green-veined White.
Source : The Butteflies of Great Britain And Ireland - Maitland Emmet & Heath.
(Vol 7, part 1 of The Moths and Butterflies of Great Britain and Ireland, Hesperiidae to Nymphalidae)