The Oath Of Allegiance?
I don't know if it is still current practice, but it was, once, necessary, on joining the Armed Forces, for example, to swear to uphold the Oath of Allegiance.
"All persons enlisting or commissioning in the British Armed Forces, except Royal Navy Officers, are required to attest to the following oath or equivalent affirmation:"
"
I... swear by Almighty God (do solemnly, and truly declare and affirm) that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth II, Her Heirs and Successors, and that I will, as in duty bound, honestly and faithfully defend Her Majesty, Her Heirs and Successors, in Person, Crown and Dignity against all enemies, and will observe and obey all orders of Her Majesty, Her Heirs and Successors, and of the (admirals / generals/ air officers) and officers set over me. (So help me God.)
Until recently no oath of allegiance was sworn by members of the Royal Navy,[29] which is not maintained under an Act of Parliament but by the royal prerogative. This is still the case for officers as, by nature of the Navy's authority deriving from the Crown and not Parliament, the loyalty of naval officers to the Sovereign is taken for granted.[citation needed]"
In it's day, would a serving Soldier be required to stand by that oath, for the rest of his/her life?