When the Great War started, Lamb hurriedly completed his medical training and became an army medical officer serving in most of the major fronts. Then he returned to art, eventually divorced, and then married the much younger Lady Pansy Pakenham (daughter of an earl) by whom they had three children. World War 2 found him as a war artist, though most of his paintings were portraits and scenes from training areas.
Aside from military subjects, the bulk of Lamb's paintings seem to be portraits, some of persons involved in London's literary scene. However, this source said that his attitudes about the Bloomsbury Set were not positive.
Lamb's painting style seldom reached very far into Modernism, though he did simplify on occasion and once in a while resorted to distortion. I might characterize it as 1930-vintage not-quite-traditional.
An example of Lamb's use of distortion.
Commissioned by the Imperial War Museum.
An example of Lamb's landscape painting.
The author. I am not sure about the color, as some Internet images differ considerably.
When Chamberlain was Prime Minister.
Painted while a war artist.
That's a Westland Lysander observation aircraft.