Thanks majm, once again I should have been more precise and said "so I shouldn't assume they're
from British
units.", which is what I meant.

There were soldiers in full uniform and also in vests, shirtsleeves and forage caps in the pictures at the two links tonepad posted in
reply #17, so I was already aware of that. The photo you posted made the forage caps much more obvious and was an excellent hint that I mustn't assume the men in the photo I posted are from British (by which I mean English, Welsh, Scottish, or Irish) units. My observation about the Prince of Wales feathers cap badge was intended to highlight my inaccurate assumption.
Regarding the quality of the image there are really only three factors involved:
1) The quality of the original photo, which was quite damaged. I don't have access to that.
2) The resolution and format at which it was scanned. I didn't scan the photo and didn't see the EXIF data so I can't be sure of the numbers here, but most scanners by default output a JPG (which is a lossy format where you can trade off accuracy/detail against file size) at something like 300dpi and 95% compression.
3) Any further digital processing. I took a screenshot of the photo and saved it as a JPG at75% compression ratio so I've made it a bit worse. That's what I uploaded.
P.S. I've been using GIMP to view this photo, so I can zoom in, adjust levels, etc to bring out detail. I'm 90% certain that what I (mistakenly perhaps) identified as something hanging over his shoulder is not caused by deterioration of the photo or digital processing.