You are subject not only to RAF regs but to the War Act.
In a tri-service establishement you are obliged to obey the orders of superiors. ONLY if you are ordered to commit an illegal act (in your opinion) may you refuse - and there WILL be consequences to that refusal.
When you have obeyed the orders you are entitled to seek redress. Not before.
If the Sgt ordered you to do something which you consider 'voluntary', even if the Doc has said you may not, you are nevertheless obliged to obey the order of the Sgt. He is your superior, regardless of the colour of his uniform. He will then be held to account. His dismissal of your objections and the countermanding of a 'clearance' from the Doc is enough to have him carpeted.
This is a difficult dilemma for you. But remember that you are not your own man; you are the Queen's man. And so is he.
I had a similar situation many years ago as a Cpl, when I was ordered by a Flt Lt to do a particularly nasty job - on my own - which should have been in my purview to have several of my guys do with me or under my supervision. He ordered me to do it because he considered that I had been insolent to him. I had not.
I did the task. It took 12 hours from 7pm til 7am the next day.
I did not like it and it was more than the usual onerous work that should have been assigned to one man, let alone an NCO. I was covered from head to foot in kero by the time I was finished.
Having done the job I simply waited for my Sqn Ldr (the SATCO) to arrive. He took one look at me and said, "I guess you have something to say, Cpl !". He listened to my points.
His action was to put that Flt Lt to work clearing all rubbish in sight around the perimeter of the base (Akrotiri) - by himself. He was at it for 16 hours and the boss drove aroung the perimeter himself to make sure he had picked up every bit.
That shight of a Flt Lt never ever bothered me again.
Agreed, IF i had been ordered to take part in the optional PT session, then I would be up on a charge/AGAI/MAA BUT he has admitted that he is unable to charge me because he failed to order me.
When I used the chain of command to question his decision to nominate me to do a voluntary sport, and the very person he recommended me to ask says that i don't need to do the sport, then he has been out ranked by a senior with a bit more common sense.
A) He suggested this person as the point of contact
B) He is not happy with that persons decision, and as i'm lower down the ranks, i'm the one getting the hassle for it.
The doctors note only came into the equation in the afternoon of Thursday, 24 hours after the decision was made that i did NOT need to take part in the sport.
I'm no fool, I understand the rank structure no matter what colour uniform is worn, and I respect each and every rank, but I was given the option by this senior to speak to a SSGT to explain the circumstances, and my next in line (the most senior in the section at that time) did this on my behalf. The SNCO gave me an option which I took, and I was able to complete the level of fitness expected of me despite the trapped nerve, and instead of facing my CO for MAA action and missing my OOA deployment, I passed the test and got abuse for it.
I was never going to win this situation really was I?
In hindsight, I still believe that i made the right decision where my career, health, and relationship with my seniors are concerned. Hopefully my RAF SNCO will see this and explain it to the Army SNCO.