It was an interesting decision by the crew to leave the autopilot engaged as the speed decayed.
Think about how long there was between the onset of the problem and touchdown; then take away the time for overcoming the initial shock and coming to some understanding of what has happened... Not much left, really.
We'll have to wait for a more detailed report to come out but, IMHO, having the AP engaged down to 200' or so (whether by decision or omission) may prove to have been beneficial.
Unfortunately, a decaying speed from about 140 knots at 750 feet to 108 knots at 200 feet, resulted in a severely degraded glide angle.
That's an interesting assertion. I believe most people think about gliding performance with the aeroplane in a 'clean' configuration, where max. L/D and the stall speed are usually quite widely separated. This is not the case when using drag flap (30 in this scenario) combined with extended gear; you will find the best speed to fly is much closer to Vs than might be apparent on first inspection.
In this instance, as the speed started to decay, a disconnect followed by flying at a speed of between VREF and VREF minus 10 would, quite probably, have resulted in a different outcome, and a more controlled landing.
Vref is calculated using a fixed multiplier and checked against limits (Vmca, etc.) - it doesn't necessarily have anything to do with glide performance. Without access to the exact residual power settings and a simulator/modelling program to try it out on, I don't think it's possible to make a call on this one. Could have been better, could have been worse...
I've flown aircraft with best glide ratios ranging from about 6:1 to 60+:1. The common factor has been that the *usable* speed range has shrunk with reducing performance, to the point that an attempt to increase speed results in a higher rate of descent and not much else.
Presumably the AP just disconnected itself at some point - maybe at an altitude of around 40 feet - when it felt it could render no further assistance?
From the AAIB report:
The autopilot disconnected at approximately 175 ft
I take it to mean that it disconnected itself, as opposed to "was disconnected", although I don't like to question the semantics of interim reports too closely as you can end up going round in circles over the exact meaning of a single word. Could easily have been taken out by the pilot.
Remember the 777 is a FBW aircraft and so the distinction between 'automatic' and 'manual' flight is subtly blurred. With the autopilot out, the envelope protection would have still been active (airspeed, angle of bank, etc.) and the automatics (even though nominally disconnected) would still be capable of making control inputs. The handling pilot would have been able to override these, albeit using higher-than-normal forces.