You must be very disappointed that neither TNA nor Red Cross could solve your problem re Bischoff.
The name rings a bell somehow, but I searched my records and couldn't find it anywhere.
There is someone here at rootschat who has posted something about someone else with the same surname, in Yorkshire -
http://www.rootschat.com/forum/index.php/topic,283590.0.htmlIf I were in your situation, and I was determined to solve the situation, this is what I would do:
I would read all the info I could find on the subject of WW1 internments and deportations, and see if that leads me to anything useful. See my bilbiography elsewhere on this site.
Ask the Anglo-German Family History Society to run a search for you in their records, if you have not already done this. You may need to join, and to pay a fee. You can also post inquiries in their newsletter.
I would research anyone and everyone with the same surname, at least the German spelling, who shows up in the UK in the late 19th/early 20th centuries; if not there, then I would look at other countries which received German immigrants by that name in that time period. I would be looking for clues that they may have come from the same family, and if I couldn't find that,
I would be looking for living descendants to interview. Usually more than one person would leave home, from what I can make out. This latter quest proved to be the key that unlocked my own family. If I had not done that, I would never have solved the mystery, as the clue that really worked was actually my ancestor's uncle, which I did not know he was at the time. Bear in mind that a lot of people left Germany intending to go to America or beyond, but did not get there, as they ran out of money or married in England and got stuck there; but their other family members may have gotten further.
Lastly, there is a German-English genealogy site where you can post inquiries. I have lost track of what it is called, but it is bilingual, and you can give it a shot. I believe the URL ends in .de as opposed to co.uk etc Hopefully someone else remembers what it is called. In my case, after I had pursued the course of action in my previous parragraph, I was able to identify a possible link on the German archival "auswanderer" records online, and then was able to find what looked like same person in online parish records, which gave me the ability to construct a genealogy for that one individual (although it did not include my ancestor). When this was posted on the bilingual site, we got a response right away from a close relative in Germany! You may need a little help from someone who can decipher a little German, if you cannot, in order to do these things.
Odds are that he somehow returned to Germany. It was not at all unusual for families to be broken up on this account. No one wanted to let on that they were German ancestry any time in the 20th century in Britain as far as I can figure out.