I don't think I ever used that reply in my life...Words and phrases are not that exact and vary with controllers and locations...
When "Cleared for immediate takeoff" I have heard: Roger, Rollin', Cleared.... and a few others...
Commands and replys need to be somewhat Random or PFE will become too robotic..
Ray Lunning
Roger, Rollin' is not correct and you would fail your RT exam in the UK if you said that. "Cleared for immediate" is relatively random in PFE and should be met with the correct prescribed reply.
Vololiberista