Not sure where it came from but you can see it here under S - https://en.wiktionary.org/wiki/Appendix:Glossary_of_British_military_slang_and_expressions#S

As for other words, I don’t think we do quite so many as the Aussies but there are words like aggro, cheapo, wino, preggo used in every day speech.

Seppo is pretty common in the UK too, particularly in families with people in the forces.