All four answers here so far have been against our subreddit rules. Remember, answers must be in-depth and comprehensive. A 1-line sentence doesn't cut it.

Took a course in British Involvement in World War 1 and was even able to go to Ypres to study a bit of the battles there. Actually being there on the field and seeing the immense graves is quite unsettling. But the part of the trip that stood out was definitely where they executed soldiers.

To put it simply, if you refused to follow orders, you would be court marshaled for cowardice and most likely sent to death by firing squad.

Now with regards to shell shock, it wasn't really understood during those times. British doctors actually dismissed shell shock as they saw it was an excuse. Famous example is Harry Far who was killed for "cowardice" and was reported to his family that he was "Killed in Action" when in actuallity he got shot by firing squad.

Another example is Eric Poole. He was a Canadian officer who suffered shell shock. He "..at times get confused and... have great difficulty in making up my mind," and went AWOL for two days. He was then tried and executed.

Here is a good resource as well.

http://www.nationalarchives.gov.uk/pathways/firstworldwar/people/poole.htm

(Dont know if this is comprehensive enough for this subreddit, but as a frequent lurker, I wanted to contribute a little.)