I'd believe that an immediate retraction to Flaps 20 at Vref might be the best plan. Sadly, the AAIB report doesn't give the next guys any knowledge.
Aerodynamic basics would state that there is no set "rule" you can derive. The higher you are, the lower the optimum flap setting would be. As you get lower the lower stall speed with more flap outweighs the drag.
It is not the AAIB's job to (re)write handling manuals:
The purpose of the AAIB is:
"To improve aviation safety by determining the causes of air accidents and serious incidents and making safety recommendations intended to prevent recurrence"
...It is not to apportion blame or liability.
Chief Inspector
They went as far in this report to analyse the FC actions as were relevant to the accident - in short, a no notice untrained for emergency, and their actions pretty much resulted in an optimal outcome:
...they kept the aircraft flying and under control so that, at impact, it was wings level and at a moderate pitch attitude
Fundamentally this is a design/engineering accident, and little to learn from the FC pov - IMHO it is impractical to expect "drills" to be produced for every eventuality - especially since in this type of accident they would need to be "memory"...
Just my 2ps worth
NoD