What was Whittle's inventive step if it was not the first practical means of jet propulsion?
The compressor, and thinking far enough ahead.
I think the
turbine part is a bit of a red herring, as far as priority goes: there were real materials problems, but everybody faced them. Whittle was good enough to design a
really efficient
compressor that was usable until the superior axial-flow design was fully developed. One less bit of radically new technology to develop.
The other thing is that Whittle foresaw speeds and altitudes far beyond the state of the art when he began. Griffith gets to be a bit of a bogey-man in this story, but it's interesting to compare their different takes on the matter.
Griffith preferred the axial-flow compressor: not surprising, since it was the long-term solution, not to mention the fact that he'd made a major discovery about the way blades worked in such a device.
Griffith was thinking of turboprops, not jets. Considering the speeds of aircraft flying and planned in about 1930, this was entirely reasonable. It turned out that the wild blue yonder Whittle was thinking about came along quicker than most people would have expected.
I think it undervalues Whittle to set him up as some unique genius, alone against (or at least ahead of) all the world. The possibilities of gas turbines, and of jet propulsion, were visible to a lot of people, but making them work depended on a lot of optimisations, and sometimes settling for good enough for the time being. Whittle seems to have got more things right here than most (including, perhaps, von Ohain, who started later, and so for whom the relevant speeds and altitudes were easier to envisage).