Lift to drag
Not an ATPL on 777 - just a one-time instructor.
Whatever the best glide speed might be, a reduction in speed can only build induced drag more and more so the overall L/D must diminish. And this will be non-linear - ever diminishing performance as speed decreases.
Perhaps the autopilot is not the crew's best friend in these circumstances.
PS. I thought landing flap was normally 25 degrees, AAIB say 30 was set.