I recall archeologists finding elite burials from ancient Britain that appeared to be sacrificial.

I don't question the presence of sacrifice in pre-Christian (or non-Christian) cultures! What I'm trying to track down is the idea that the important part of the ritual is the consenting, the willingness to make the offering—whether it is one's own life or some other precious thing—without which the physical act of sacrifice has no strength. It seems to be taken for granted in both real and fictional mythologies and I would like to know how far back it goes. Elizabeth Goudge being a Christian writer, I'm especially curious if there's a theological precedent of which I'm not aware because I'm neither Christian nor a religious scholar.