Jay wrote:

How valid is the "pom" label toward the English?

What do you mean 'valid'? Its what Australians call English immigrants.

I've been told that:
a) It's supposed to be POHM (Prisoner of Her Majesty) and
b) The actual label is supposed to refer to Australians (more specifically
those who are a direct descendant of convicts).

Wrong. The most likely etymology is that it is a contraction of
"pomegranates", a former rhyming slang term for "immigrants".