When I first saw this on the front of a book catalogue from 1975 (number 9 from Elysian Fields Booksellers) I thought it was a reproduction of a photograph. Obviously, on closer inspection it is a drawing, described in the catalogue as:
"a previously unpublished drawing by Edward L Van Someren done about 1897. He was the son of Ivy Van Someren, the woman Baron Corvo knew and stayed with during his stay in Venice. He is well known for his paintings of victorious generals and admirals of the First World War"
Well, if he is that well known no one seems to have told Google about it. I can't find even a single reference in the National Portrait Gallery catalogue. Which is a shame because one sniffs the faint odour of 'a story' behind this. The catalogue doesn't say that the drawing was for sale.