from National Portrait Gallery.
Askew Codex (Codex Askewianus) did get it's name according to Dr. Anthony Askew[1] (1699-1774), a famous book collector from England, that had assembled a valuable collection of books and manuscripts known as Bibliotheca Askeviana[2]. After death of Dr. Askew the collection was sold in an auction during 1775. Anyway, the Askew Codex is not on the list of books sold in the auction, but it „was bought by British Museum in 1785 from Dr. Askew heirs“, as wrote G. R. S. Mead[3] in his introduction[4] to his translation of Pistis Sophia. It is not clear, from where Dr. Askew had obtained this codex. It is written in perfect Sahidic dialect of Coptic language on a parchment generally containing 356 pages, since every page has about cca 30 lines. The entire work is usually called as Pistis Sophia.
There are only few editions of the Coptic text of Pistis Sophia. Some of them are presented above. The Petermann's edition is freely available from the internet[5] and it is of high quality.
There are several versions of Petermann's edition of Pistis Sophia on the internet, but some pages are missing in each of them. Here is a compilation of available versions of this document containig all pages of Coptic text.
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[1] Dr. Anthony Askew by the painter Thomas Hodgetts
from National Portrait Gallery.
[2] The title page of Bibliotheca Askeviana catalogue. 
(entire document on Internet Archive)
[3] G. R. S. Mead (1863–1933) 
- expert on Hermetism and Gnosis, translator and author of many theosophic works, founder of the Quest Society.
[4] Pistis Sophia translated by G. R. S. Mead - INTRODUCTION
[5] Petermann's edition of Pistis Sophia on the internet - Internet Archive | Google Books