Wow, Thank you for your question. I've just realized I have been doing head nods for years without thinking about it! Oh no!
To help you, it's like what a lot of others have said, I do agree that a single (or maybe rarely double) up-to-down nod is one of agreement or acknowledgement of a proposed course of action (like in a meeting or something).
Starting from down, then up, once, is used in lieu of 'hello'. I've been thinking about this, and even if you don't verbalize " 'S up", the way it is annunciated seems to naturally bring your head up, from the formations of the 'S', and up a bit more when you say the 'up' part.
This head nod is best used in those situations with strangers or acquaintances when neither of you knows who should or will go first. Approaching from opposite directions in a hallway is a perfect example. You almost want to communicate that you are a guy going about your business, you are busy in your own thought process, but you are showing respect by breaking out of this momentarily to acknowledge the other guy, a sign of mutual respect.
Never smile at a guy, unless he is gay, and you use the smile to communicate friendship, and to show you to others that you are secure in your masculinity. This says, 'this guy is my friend, and I don't give a f$&;: what you think or infer from my smile'
Whether you are interested in them or not, always smile briefly at women, but not creepily.
I have spent enough time in the United Kingdom to know that the nod doesn't work at all in the same way it does in the States.
Gosh, I had no idea there was so much to this.....