This article indicates that E. M. Forster is also present at Gregson's party which includes Virginia Woolf. Has anyone managed to pick him out in that scene? He would have been 42-43 at the time, and most of the men look too young.
When Edith arrives in London, the actual shooting location is St. Pancras. Is it intended to be St. Pancras in the story too, or is it standing in for King's Cross?
True, and appreciate the info, but it is not without precident. In the Harry Potter series, the Hogwarts Express originates at King's Cross too, however the only external shot (Chamber of Secrets) is of St. Pancras. Secondly, in the 1920s, wouldn't any train from Yorkshire come into King's Cross (NER/LNER), while the LNWR / LMS would go into St. Pancras from areas including Manchester and Leeds? So one would wonder how it was that Edith ended up on one of those lines.
Virginia Woolfe seems reasonably passable, she too would have been about 40 at the time. The Prince of Wales looks approximately the right age at any rate and is no worse in appearance than Guy Pearce in "The King's Speech".
This guy, if that is who he is supposed to portray, isn't even close on points including apparent age, hair color / style, facial hair, height, etc..
Also, looking back over your words; the St. Pancras station was filmed at because, though the Hogwarts express leaves from King's Cross, J.K. Rowling didn't describe King's Cross, but another station as King's Cross doesn't have the stations that she described -- not in the way she described them anyway, so filming at King's Cross was impossible and St. Pancras stood in.
Here is the story on the stations. J.K Rowling confused King's Cross with Euston station as she was living in Manchester at the time. The use of St. Pancras for external shots however, was at the preference of the film crew.