The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.
The usual jet airliner L/D of 18 is for the clean configuration and I suspect that flap 30 yields substantially less. A simulator run would present the raw F30 L/D, but in any case we did have some thrust and the achieved slope and L/D is derivable from the FDR.
There are a number of flapped gliders that use flaps to add drag. The big caution with flapped gliders is not to add too much flap as you can lose considerable altitude reducing flap. Many glider approach accidents have happened when flap was reduced with insufficient altitude.
I would not want to explore in the air how the 777 with thrust restricted to the accident setting reacts to a flap reduction attempt at 600', but it's possible the AAIB will decide to investigate that in a simulator.
In a high drag situation, increasing airspeed increases drag substantially. With flap 30, the best L/D speed would be lower than with lesser flap selections.
With gliders that have powerful spoilers and/or flaps, you can select full spoilers and/or flaps full on and if you are still not coming down steeply enough, add airspeed to steepen the slope.
In this accident, we see a trade of airspeed for glideslope, mostly at the behest of the autopilot. Given the touchdown point and the 108 kt. cited by the AAIB, they were amazingly lucky.