"If the British Government should shred the passport of anyone who has gone to fight for a foreign state, surely that should according to your logic, also apply to any government anywhere in the world"

Why Raggy? The British Government introduces, debates and passes into law legislation that only affects the British people and those living within the country. What other nations and the nationals of other nations do outside our jurisdiction are actually beyond our control. So taking your example of Nepal. If the Nepalese Government enter into a treaty with the UK, with India and with Brunei that allows it's citizens to volunteer, compete for, and gain places to train as soldiers to serve in the armies of those nations there can be nothing illegal, or morally wrong in anything done by the Nepalese Government or any Nepalese national.

The reason Rhodesia was mentioned was historical and fully explained in the section I pasted. Highly relevant it showed respect and regard for the Head of State while UDI showed utter contempt for her British Government.