I'd probably go with the second option, though I'm not sure there would be many interesting SAS moments in WW2 that involved sewers. Unless I consider some other faction during the war, or perphaps an "alternate history" scenario where a SAS team helps French resistance fighters pursue Germans along several places. The sewers could be in Paris (the resistance used them for bases and transit) and serve as a way to another level.
Lately I've been thinking about an alternate history or "what if?" scenario where one would control either four SAS or 101st Airborne trying to capture Göring's art train. Unfortunately I can't find any images of what the tunnel was like in 1945, so I'd have to be pretty imaginative there
