Pom/pommie/pommy are not acronyms. Nobody knows for sure what the etymology of �pom/my/mie' is, but neither the Oxford English Dictionary (OED) nor the Australian National Dictionary (AND) even mentions the idea that it might have anything to do with prisoners or acronyms involving prisoners - eg from Prisoner of Mother England.
Here are a few key historical facts...a) 'Pommy' appeared nowhere in print before 1915 and b) �Pom' then appeared four years later. Both meant �a British soldier'. c) If �pommy/pom' had anything whatever to do with prisoners or acronyms, why did theses words not appear on paper anywhere until 130 years after Australia became a penal colony and about three generations after the last convicts were sent to New South Wales?
Both the OED and the AND say the source is obscure, but suggest �pommy' might be associated with 'pomegranate', a concept first outlined in 1923, within a decade of the word's first appearance in print. The OED claims this to be (quote): "the most widely-accepted" etymology, which makes sense for two reasons...a)pomegranate very roughly rhymes with 'immigrant' and hence, "immygran(i)t/pommygranate" was possibly a jokey catcall first used by schoolboys - and b) the pomegranate is a bright red fruit resembling the sunburnt skin of newcomers to Australia.
Unfortunately, neither the OED nor AND is available free online, but if you click
http://www.worldwidewords.org/qa/qa-pom1.htm you will find a reliable web-page on the matter. It was produced by the noted etymologist/lexicographer, Michael Quinion. He, too, dismisses the acronymaniacs' ideas, so it is pretty clear - despite there being no total proof - that �pomegranate' is the way to go...Forget the convicts!