This relates primarily to the UK and how the next of kin of those killed in action in the Great War were notified.
For years I had believed that it was done by telegram and that people would hate the sight of a telegram boy appearing in their street. However recently I read that the news was actually delivered by letter except if the person killed was an officer.
I have seen a few episodes of “Not Forgotten” where they talk about the telegram- when produced it is clearly a letter. (I am having a few doubts about that series but that is a different matter- it is done in the best interests I think).
It appears that immediate notification was usually by telegram, with further communication by mail. These included a memorial scroll, a metal plaque, and a “personal” short note from George V.
My understanding was usually that it fell to the local Vicar to inform the next of kin, and hence they would often become a pariah as the local women were so frightened of a visit by him, and what it entailed.
From talks I had with my grandfather years ago, I learned that commanding officers were expected to write a letter to the family of a soldier in his command. My grandfather said it was his saddest duty. He said at times his letter beat the official notification.
Regarding the telegram delivery (once again in the U.S.), my grandmother said that in many small towns the normal telegram delivery boy was not intrusted with those telegrams. The head of the office took it as his responsibility to make that telegram delivery himself. I would be willing to bet this was the case in much of G.B. as well.
I went through the files of two of my grand uncles today (both were killed in France in WW1). One of them was wounded five times before being killed in the last major action his unit fought- when wounded in 1917 his mother found out from reading the newspaper and had to send a telegram to the relevant authorities to find out if he was okay. I imagine it was rather patchy.
An example of a WW2 notification telegram can be seen here http://webpages.charter.net/ocotillo/pages/Telegram%20page.htm. There were sometimes two notifications - an initial “believed to have been killed” with a later confirmation one way or another.