Look at your employer’s T&C and see if additional leave is granted for recognised youth groups / voluntary work and whether it is paid or not. Not sure we can go for the military angle anymore, although that’s how I get it and have done for many years. I just make sure I say I’m using for cadets.
This will let you know in advance how easy / difficult the discussion will be.
TBH the letter is a waste of time. If you’re unsure go and see personnel and ask the question.