Joe,
Burrard writing in Volume 1 of," The Modern Shotgun," First published in 1931,states that," Whitworth steel barrels were still in use but meeting increased completion from Vickers and others who had introduced steel barrels of comparable strength and performance."
In the 1980's my friend the late Geoffrey Boothroyd attempted to establish the date that the manufacture of Whitworth barrels ceased. Despite researching the issue with the history section of the Manchester central library, Geoffrey was unable to ascertain the date when production of Whitworth Fluid Compressed steel was discontinued.

Roy Hebbes