In the account of Winston's dream, the text doesn't actually say that the dark-haired girl was "throwing aside her uniform" but that it was "thrown" aside after, she, in "with what seemed a single movement ... tore off her clothes and flung them disdainfully aside." The text tells us explicitly that the dream woman's naked body aroused no sexual desire in Winston, but that he was deeply impressed with admiration for the gesture, which seemed to him to erase the whole culture of the Party and Big Brother with one sweep of an arm. The uniform she discarded without a second thought symbolizes the Party and all its drab repression, its crushing of people into one "uniform" abject mold. For Winston, it was the assertive way the uniform was torn off that became a symbol of defiance and rebellion.
It's also significant that Winston feels no sexual desire in this dream. Although the dream woman foreshadows Julia, at this point, Winston is still a product of incessant Party propaganda. He wishes deeply to rebel--and he does--but he is still caught in the Party's sexual repression, channeling his energies into emotions like hate and fear, fantasizing violence rather than love towards Julia, whose name he does not yet know, but whose anti-sex league sash and very demeanor fills him with rage. It will take developing a relationship with Julia to reignite his ability to feel compassion and healthy sexual desire.
In Part One, Chapter Three, Winston dreams of a dark-haired girl. In this dream, the girl "tore off her clothes and flung them disdainfully aside" while walking across a field.
In doing this, the dark-haired girl casts off the Party's control over her body and her mind. She is openly rejecting their rules and her 'disdain' sends a message of total defiance. She has had enough of living under a totalitarian regime and, by walking towards Winston, is signalling to him that she is ready to rebel.
For Winston, this act is symbolic of his repressed desires. Readers know Winston wants to rebel against the Party but is hesitant to do so. The dark-haired girl, therefore, represents Winston's inner confidence; it gives him a push to follow his heart.
This dream is also important because it foreshadows the future relationship between Winston and Julia, who support each other in rebellion against the Party.
I think this represents a couple of things.
First, you ask about the symbol of throwing off the uniform. I think this symbolizes throwing off the Party from one's self. She is letting go of the exterior that she has to maintain to reveal the real self on the inside.
Second, I think this action is very literal too. Sex is frowned upon by the party, and Winston allowing his mind to go to the place to actually long for it is rare, but human. Winston is being a normal and very real man imagining a woman undressing.