This Great Mormon is described as being, "...a pure bilateral gynandromorph. One half of the butterfly is female, with paler colouring and flecks of blue, red and tortoiseshell. The other half is male, with darker colouring."
Gynandromorphy itself is not all that much of a rarity in nature, however, "Pure bilateral gynandromorphs are incredibly rare," according to the NHM's butterfly house manager, Luke Brown.
There are actually a few things that can lead to this:
"Bilateral gynandromorphy results if this error occurs during the first cell division, resulting in an insect that has male cells on one side and female cells on the other.
Gynandromorphy can also occur when an egg with two sex chromosomes, instead of the normal one, gets fertilised by two sperm."
If you check out this (brief) article itself, the author goes into a bit more deal about the museum's excitement over their unique Great Mormon butterfly, gynandromorphy and the "Sensational Butterfly Exhibit" in which this guy/gal is exhibited.