Regeneration is set in a wartime hospital for World War I officers recovering from wounds and, mostly, "shell shock," or what we would call post-war traumatic stress syndrome (or PTSS). Though focusing on officers, it addresses the experiences of soldiers from every level of social class because part of the psychoanalytical "regenerative" [experimental] therapy Dr. William Rivers practices is to induce the officers, sometimes through shocking means, to relive and describe all the details of their war experiences. In so doing, the reader is taken back to the trenches to witness the horrors of the men whom the officers had charge over. So, even if an officer is the one who sees a ghastly casualty he tells about, the experience incorporates the lower class soldier(s) who is the victim of enemy fire or mortar attack.
One of the most telling thematic points Barker makes regarding social class is the disparity between what officers are allowed and what soldiers are allowed for the same behavior, events or choices. The most central means of expressing this theme is through Rivers' Craiglockhart War Hospital in Scotland. First, note that all officers are assumed to be upper class except for the few who have risen by means of valor, and these few are treated as though also upper class officers. To show the disparity, Barker demonstrates that when officers suffer shell shock and are unable to go back into battle, either because they have hysterical injuries (with no biological basis, like blindness in functional eyes) or because they hysterically refuse, these officers are sent to Craiglockhart Hospital for treatment. In glaring contrast, when lower class soldiers suffer the exact same afflictions of shell shock, they are court martialed for being cowards and thus traitors, with the maximum penalty being death.
For the most part, it is the upper social class of Britain that are explored in Regeneration. Why? Because Siegfried Sassoon is a young aristocrat, Wilfred Owen is a Company Commander, and Dr. Rivers is, well, a doctor. I suppose you could say that the upper social strata of Britain was not spared the horrors (and specifically the horrible psychological effects) of World War I.
Let's look at Siegfried Sassoon as an aristocrat and officer in the British army. He is held more for political reasons than anything else. Why? Because he publishes "A Soldier's Declaration" which says (among other things):
I am making this statement as an act of willful defiance of military authority, because I believe the war is being deliberately prolonged by those who have the power to end it.
This type of statement could have gotten this army officer shot, but it is his social class that keeps the government from doing so. Not only is Sassoon an aristocrat, but he is also considered a war hero. It was much safer, then, to just declare him unsound mentally and send him to Dr. Rivers.
Also note that Dr. Rivers, as a doctor, is also seen as a higher class person. He gives us all of his opinions through those eyes. It is interesting, of course, that his thoughts continually remain with Sassoon and sometimes with Billy Prior; the lower social strata folks just aren't as interesting, I guess.
Billy Prior is also an officer, a second lieutenant, who is also considered higher than the common man on the social ladder. That's why it's so interesting to find him falling in love with Sarah Lumb, who likes to talk tough and drink hard as she works for munitions. Even though this would be a marriage that transcends social class, it just might end happily (as long as Billy Prior survives the war).
Thus, as you can see, even though the higher social classes are the focus, the self-reliant lower classes are represented through the awesome female character of Sarah Lumb. All in all, there isn't a single class left unaffected by the trench warfare of "the Great War."