Bouncing-Bomb Man by Iain Murray (Haynes 2009) p129:
"were dropped from 18,000'.... average striking velocity 1100fps"
A Hell of a Bomb by Stephen Flower (Tempus 2002) p98:
"from 18,000'......had taken 37 seconds to fall, impacting at just over 1,000 ft per second."
It would be nice to see the original test reports, I wonder if they are accessible anywhere; IWM or perhaps the Brooklands museum.