I wonder if there were simply not so many towers in the UK, and more of these underground rooms? That might explain why there are so many examples from the Czech Republic, Germany and Latvia, and why the British ones are harder to find. Still, here's some more:
"the renovated Norman Tower; the Powder Tower, ..."
Windsor castle By William Harrison Ainsworth http://www.google.co.uk/books?vid=OCLC0309083...
"About equally distant on the road to Concord was the Powder Tower, whose remains today are one of the most characteristic relics of the past near Boston."
The Memorial History of Boston: Including Suffolk County, Massachusetts. 1630-1880. Ed. by Justin Winsor
J. R. Osgood and cohttp://www.google.co.uk/books?vid=OCLC0097815...
I hadn't heard of Pulvermagazin, which is obviously a more direct translation of "powder magazine" - Duden says it only applies to ships (and guns) but I'm suspicious of that:
Pul|ver|ma|ga|zin, das: vgl. Pulverkammer (1).
Pul|ver|kam|mer, die: 1. Raum auf Kriegsschiffen für die Lagerung der Munition. 2. (Milit. veraltet) Raum in einem Geschütz für die Ladung.
© 2000 Dudenverlag
Grimm's also has:
Pulverkammer 2) bombensicher eingedeckter raum zur aufnahme des pulvers und der kartuschen hinter den belagerungsbatterien; auch die zur aufnahme von pulver bestimmte oder damit gefüllte minenkammer. mathem. lex. 1, 1054. EGGERS kriegslex. 2, 505.
PULVERMAGAZIN, n. locus concameratus ubi pulvis pyrius conservatur FRISCH 2, 74a: feuer ins pulvermagazin! SCHILLER 2, 292 (räuber, trauersp. 4, 8); das ganze pulvermagazin sprang in die luft. PLATEN 4, 163.
(both above: Deutsches Wörterbuch von Jacob Grimm und Wilhelm Grimm. 16 Bde. [in 32 Teilbänden]. Leipzig: S. Hirzel 1854-1960. -- http://germazope.uni-trier.de/Projects/WBB/wo...
Still, I'd plead for:
Pulvermagazin = (gunpowder/powder) magazine
Pulverkammer = (gunpowder/powder) magazine
Pulverturm = (gunpowder/powder) magazine/tower
i.e. both "magazine" and "tower", as the term is more common in English (as common as Pulverturm in German) and some of the "magazines" (e.g. in post 6) are built above ground.
More images of "magazine" built above ground in Williamsburg:http://www.vahistorical.org/onthisday/magazin...
Powder Magazine in Williamsburghttp://www.ohlone.palo-alto.ca.us/Williamsbur...
This octagonal building is the Powder Magazine where the arsenal of the colonial militia was kept.
and in Cornwall:http://www.cornish-mining.org.uk/sites/images...
Wheal Clifford powder magazine