I came into some of my great-grandfather's possessions and found this photo of what, as best I can tell, is a stamp from Stallag XVIIIA, a POW camp. Unfortunately I can't identify much more about it. Interestingly, nobody in the family has ever heard mention of my great-grandfather being a POW. His name was Robert Graney, and he was from around Slough, England. The "POW camp stamp" writing at the bottom is almost certainly my grandmother's handwriting, so presumably is from significantly later than the rest.
If he was PoW it should be listed in his service record which you can obtain if you have permission of next of kin or he died more than 25 years ago.
https://www.gov.uk/get-copy-military-service-records/apply-for-someone-elses-records