Additions to Bibliography:
Dewey, Peter. Farm labour in wartime: the relationship between agricultural labour supply and food production in Great Britain during 1914-1918, with international comparisons, University of Reading, 1978. (internees were sometimes employed as farm labourers.)
United States Foreign Office (ed.). Correspondence between His Majesty's government and the United States ambassador respecting the treatment of German prisoners of war and interned civilians in the United Kingdom, London: H.M. Stationery Office, 1915.
United States Legation (Great Britain) (ed.). Reports of visits of inspection made by officials of the United States Embassy to various internment camps in the United Kingdom. London: H.M. Stationary Office, 1916.
Paterson, Sarah. Tracing Your Prisoner of War Ancestors: The First World War, a Guide for Family Historians. Barnsley, South Yorkshire: Pen & Sword, in association with the Imperial War Museum, 2012. The author has worked at the Imperial War Museum for some time. This book covers both military and civilian internees. See especially chapter 10 and Appendices. Appendix 8 contains a 23-page list of known internment camps in the UK, although details are very sparse. These named camps may have held military and/or civilian internees.
Includes a few references and/or quotes in relation to specific internees:
Mustafa Mehmed (letter),
George Kenner (ID card with photo, plus a lot of other info),
Julius Bloom,
Harry Hermes,
Benj Cseh,
Carl Hans Stielow.
Some highlights from Paterson's book:
The Times, 17 Jan 1919, page not stated: a list of 259 persons who had been exempted from repatriation or internment. I presume, but am not sure, that, at this late date in the war, they had probably been previously interned and were only now being exempted after due process and investigation.
Barry Chinchen. "Aliens, Internees and Prisoners of War in the UK, 1914-1920". This is a folder containing copies of relevant material. I believe it is held at IWM, but not sure.
Society of Friends library - presumably in London - appears to have numerous archival items.
British Red Cross Archives - presumably in London; different holdings than ICRC.