A pleasant afternoon on the Thames
I don't know how accurate this is but on a sunny Sunday afternoon having boarded the London duck by the London eye we proceeded along the Thames in a ship shape and satisfactory fashion. Our on board guide informed us as we passed moorgate that the origin of the word "POM" came from the ships anchored in the Thames, waiting to take the unfortunate lot to the other side of the globe. In order to identify the ships when the prisoners were removed from moorgate prison they had POM painted on the side of them to signify Prisoners Of Moorgate...POM. I have no idea if this is the correct answer but it is a nice story.