YMB, The nickname 'pommy/pom' was created by Australians (almost certainly) on the basis that new arrivals from Britain got rapidly sunburnt and took on the bright red hue of the pomegranate. No, it doesn't have anything to do with prisoners! Accordingly, why would anyone find, "My goodness, aren't you sunburnt!" as being particularly offensive?
Similarly, the nickname 'limey' came about when American seamen discovered that British sailors were made to eat limes in order to ward off scurvy on long voyages. Again, there is nothing offensive whatsoever in the name.
The French 'rosbif' is nothing more than an acknowledgement of the British love of 'roast beef'. Utterly inoffensive again, so nicknames ABOUT us are mild.
Why is it then that, since rhyming slang is basically a Cockney/English habit, so many nicknames for nationalities created BY the English are clearly - and deliberately - offensive?