i dont know much about other uniform bodies but I myself was a scout.
I will have to say that if you're looking for serious leadership training, scouts would be your best choice. At the beginning, it might be very tiring and "torturing". Probably when you're in form 1 - 3. That i would refer as the "training and following" stage. Later on, you'll proceed to the "leading" stage, where you'll get involve in the process of planning and running events like camps, combine meets or even campfires.
But then again, whether or not you get the chance to lead all depends on you formative years, meaning the "training and following" stage (form 1 - 3). If you're hardworking, responsible and can show your committee members that you're capable of leading and being persistent, then you'll flourish and oportunities will start pouring in.
I myself was very fortunated as I was given lots of opportunity to lead various events and people. And that had helped me alot. It had made me more mature and also gave me various leadership skills which will indeed come in handy during stressful times.
Also by joining scouts you'll be able to learn to be independent, aswell as various useful skills and knowledge. Just remember not to stop getting involve. Join every single camp available and try learning from your seniors when ever possible. It takes time but the fruit in the end is indeed sweet.
Lastly, remember that before you can lead, you must learn to be lead.
As for other uniform bodies, don't know much but you might want to consider entering the military band. By the end of form 5, at least you can play an instrument or 2 eh? No comment about the others though.
But have to say that whichever uni unit u choose to join, the most imporatnt thing is to get involve and stay active. Not to ponteng here and there