The Imperial War Museum has a fairly large Photograph collection covering the "First World War (Being a doughboy would suck ass)":http://www.iwm.org.uk/collections/photos/WW1album.htm, the "Second World War (and being in D-Day would also suck ass.)":http://www.iwm.org.uk/collections/photos/WW2album.htm, the "Falklands War (well, I'm not British)":http://www.iwm.org.uk/collections/photos/falklands.htm, and a generic "conflicts from Post-1945 (yeah, because there wasn't many wars after WWII)":http://www.iwm.org.uk/collections/photos/post45.htm.
A fairly cool collection. Some nice quality photos.