Findmypast.co.uk recently added files for WWI and WWII prisoners of war
The notes for WW1 say they are for 7,703 British Army Officers who were Prisoners of War between 1914 and 1918. However, the first person I checked was a Canadian, G. W. Northwood, and his record is included. He`s listed as rank: Capatain, service: Infantry Canadians. The additional information available by pay per view or subscription is Section (meaning where serving); date missing; and Interned (which gives the country and what appears to be the date of repatriation).
For WW2 there are three sets of records:
Prisoners of War 1939-1945 – British Navy & Air Force Officers: 19,229 records
Prisoners of War 1939-1945 – Officers of Empire serving in British Army: 39,808 records
Prisoners of War 1939-1945 – British Army held in German territories: 107,000 records
The records will usually provide you with the following information about your ancestors: name, rank, regiment, army number, camp number, Prisoner of War number, camp type, camp location and extra notes, where applicable.
The first record I checked, for A.M. Reid in the British Navy & Air Force Officers series turned out to be a Canadian airman interned at Stalag 334 at Lamsdorf. However, the other I checked were British.
The source given is Naval and Military Press Ltd so you might want to use these are clues in looking for original records.
It`s worth checking this source for Canadian POWs despite the title.