Here in the USA, "Assault" is the act of making someone fear that they might be attacked; it doesn't involve any sort of contact. Someone yelling in your face who might be able to hurt you, someone brandishing their fist or a stick, anyone making you have a reasonable fear that you might be attacked is committing "Assault".
In the USA, this woman would have been charged with "Battery". USUALLY it comes along as "Assault and Battery" together, when people have words and then one of them attacks and hits the other.
But in this case, since she came up from behind before you saw her and hit you, that's "Simple Battery".
Still actionable, and here at home, I would have gone to the police just to let the nasty b!tch and her ilk know what she can do and what she can't do, since (being a woman) if I touched her I would be in more trouble than she would.
If it were a dude, it would have been different .....