The flaps were retracted to Flaps 25 at Flaps 30 Vref -17 or 18 kts.
Retracted flaps at Vref -17 or 18 kts probably does not increase gliding distance due to the increase in angle of attack.
You can check this in the sim - fly level flight at Vref Flaps 30. Note your fuel flow. Then do the same at Vref -17 or -18 with Flaps 25. Note your fuel flow. The higher fuel flow would indicate which configuration has more drag.
I've been told L/D for configured Boeings is typically Vref +10(an estimate from a test pilot, depends on flap settings and different model aircraft). If correct you're in the back side of the power curve and decreasing flaps decreases your gliding distance.
Hopefully the final AAIB report will cover this subject.