I've just been reading an article written in 1915 by David Prain (who also like myself took up botany at the University of Aberdeen
), entitled "Some additional species of Meconopsis". I straight away thought of this post about white M. horridula, because in the article Prain describes a new species of white flowered thorny Meconopsis, which he called M. decora, from the east Himalayas:
"This seed is said to have come from the Abor Country, and Mr. Hay on raising plants had supposed them to belong to M. aculeata; when flowers appeared he took the form to be a white-flowered variety of that species. It is, however, in reality so distinct that it is advisable to treat it as the type of a separate group."
There's not a great difference between all the species of the horridula aggregate (and many in cultivation are mis-named), so yeah, the plant we're looking it could also just be a white variety of any of them.
As for the fate of Meconopsis decora, its species level classification was whisked away by Taylor, who assigned it the status of a hybrid. Though interestingly Taylor also admited he couldn't explain how these 'hybrids' could arise spontaneously in gardens and in the wild.