A very common difficulty is caused by the fact that lists of POWs at various camps in Britain were not generally retained in the country after the war. However, one resource that’s well worth looking at – if only for background information on the camps – is the excellent series of After the Battle magazines and books: www.afterthebattle.com
If you go to the above website, click on the tab “Index of Issues” and you’ll discover a downloadable, searchable index of every After the Battle issue (dating from the early 1970s). If you’re very lucky you just might find your ancestor mentioned by name! For example, Hans Teske is to be found in Edition 17, page 53. Back numbers of past issues are available through the website. (Teske was, officially, Britain’s last POW of WW2, since a clerical error prevented him being discharged from prisoner status after the war and, technically, he was therefore still a prisoner until his death in the year 2000.) While you would be very lucky to find the very person you’re searching for in this way, remember that it’s a bit like the lottery … you might just win! Search under the name of a camp where your ancestor was detained and you have a good chance of finding some details of the camp itself at least .
If your ancestor was in the Luftwaffe you should look at the excellent After the Battle volumes: The Blitz Then and Now (3 volumes), and The Battle of Britain Then and Now. These record virtually all German aircrew who were taken prisoner, wounded or killed during these air campaigns: they can also be purchased from the website, and are available in some libraries.
In the future I’ll be adding more info about tracing German POW ancestors, so do try again soon if you don’t see what you are looking for today.