Manslaughter isn’t really a “halfway house” in the way people sometimes think. In UK law the difference usually comes down to intent and circumstances, not simply whether someone died.
With murder, the prosecution has to prove there was intent to kill or cause really serious harm.
Manslaughter covers situations where someone unlawfully caused a death but the legal test for murder isn’t fully met — for example loss of control, diminished responsibility, excessive self-defence, or situations where intent is less clear.
That’s why you sometimes see someone deny murder but admit manslaughter, or vice versa. A stabbing can still legally end up as manslaughter depending on what the jury believes was happening at the time.