Dick,
Prisoner of Mother England as an origin of the word Pommie is unlikely in the extreme.
I would be dubious about any etymology that says a word entered the English language as an acronym.
It doesn't seem to have happened until the mid 20th century when things like Radar and Laser came along - and of course there's SNAFU, FUBAR and others of that ilk.
Back to pommie - nobody knows for sure, but for a more reasonable explanation, see what Michael Quinion has to say at World Wide Words.