It’s a good question about where it should have fit, and I think that answers why we didn’t see it.
Tacked onto the end of Angels it would have been “two” endings when the important one was Amy’s departure it would have risked diminishing that.
The fact that it was written by Chibnall and given it’s length, I wonder it if was meant to be Pond Life 5
Sad they didn’t film it, it could have gone on the DVDs. I think they’ve only released it because the one reaction I’ve heard to Angels universally is “What about Brian?”
it’s a testament to the actor, and perhaps even the writer, that in just 2 episodes, Brian Williams grabbed such a prominent place in the audience’s heart that the production team had to release unfinished material to satisfy the audience they accidentally left wanting.