It is sad that these days it is very difficult indeed to procure genuine British historic specimens. While fraud has always been rife in entomology it seems to be more common than ever as dodgy dealers cotton on to the fact that large sums of money can be made by trying to pass off European specimens with fake data as genuine British historical taxa. I have over the years been fortunate to have good friends who have access to the real thing and were kind enough, for a small fee, to let me have them. All serious collectors should make a point to name and shame all those dealers who are known to participate in such shameful practices polluting private collections with their dishonesty to make a few quid.
Aporia crataegi, extinct as a British resident for almost 100 years, 2 examples from Kent 1885 and 1907.
Finally for now a species which is easy to fake, specimens have been seen regularly in the south of England over the past few years, this male was taken by my great friend T W Harman FRES and therefore greatly treasured by me a couple of weeks before his passing, Kingsdown Kent, September 2015, he took a series of 13 which have now found their way into various private collections and institutions.
I had no idea that N. polychloris was no longer naturally occurring. What killed it off in Britain? It is a large-ish obvious butterfly which like most Nymphalids should have been of pretty hardy stock. Just would not expect something that size and probably gregarious to of disappeared.
Whereas, lycaenidae and a fair number of hesperiidae are known to either have complex life cycles (needs) or are localized due to habitat requirements.
A terrific species to have lost in Britain. What a shame...
I an afraid the answer is that we nobody really knows why Nymphalis polychloros became extinct in Britain. Various reasons have been put forward but it may have been a combination of factors. The same with Aporia crataegi, although many believe it was climate change because attempts to re-introduce it failed. This is a nice array of extinct and a rare British species that would be very hard to source today and are always nice to see.
It i was starting out collecting now I think I would try to catch myself as many species as possible as I would control the data and then leave well alone, finding a trusted source for specimens is now a thankless task and for some rarities now you would need very deep pockets as they can only be obtained from old collections.