Are you taught that uchi is a cut? Thats interesting as I've been taught that uchi is really more of a strike than a cut, which is kiri.
Shomen uchi, as I'm taught, is a strike (usually to the top of the head, or forehead)
thereby "knocking" the head back and exposing the throat.
Shomen kiri is a cut to the head. In my experience, the cut starts at the front top of the head and travels down ending at various places, depending on what your sensei is teaching you.
I know that when sensei asks for shomen uchi, I try to make contact on his forehead knocking his head back.
When he asks for shomen kiri, I step in and cut from the top of his head clear through his hara.
The energy is traveling in different directions depending on whether it's uchi or kiri......