Bit of a rambling post here but bear with me, all errors are mine :)

Much though I would loved to have seen this majestic construction rotating gently in the wind threshing and winnowing the corn, I have some reservations about its purpose, construction and evidence.

WHY?
Fields at the time (Stonehenge IIIA 2.000BCE) were small, used for growing Emmer, Einkhorn, Barley and Spelt for domestic consumption, bread and ale. Production was small and localised. Salisbury Plain wouldn't have looked like the Great Plains of the US. It would be like these days constructing a vast processing and distribution centre for the output of a couple of allotments. Threshing and winnowing doesn't need a construction 20+ foot high, it was almost certainly done then, as it still is now, at ground level.
Most of Salisbury Plain seems to have been pasture for cattle, sheep, goats and lots of pigs.

HOW?
Let us think about the constructional materials....Big ropes for dragging and pulling stones upright must have been known but this construction needs lightweight high-tensile stuff to work. When the much larger scale model is constructed, (using, of course, natural materials available at the time and using known techniques) and the ropes have been tensioned, soak it in water, (a typical UK summer!) and watch it collapse under its own weight. The sails are equally a huge problem. Are we sure that sails of this sophistication were ever known at this period? What were they made from and how, woven? And the evidence? As above, wet sails weigh a tremendous amount. Boats were generally rowed.True windmills had to wait until the very early Middle Ages.

EVIDENCE
Transportation of the un-threshed corn. There seems to be little or no evidence that heavy-duty wheeled carts were about in this period. Horses had not been domesticated and why drag the light but bulky un-threshed corn even a mile when you could do all the processing, including grinding, on your own doorstep? Carts or a travois would have left deep ruts in the thin soil over the 10 centuries this is supposed to be in use.

PRACTICALITY
OK, so you have this stationary mill 20ft in the air, you climb up the ladders (?) and place the sheaves on the top of the horizontal lintels.
Climb back down and release the brakes (?). There is a lot of inertia up there, a bit of a push start? Don't know about you but I wouldn't want to be anywhere near it when it starts spinning! The wind springs up and all the sheaves are blown off the top! You really can't have anyone up on top manning the "rigging" as you are adding to the weight.
IF you get the thing going, the wind will tip the whole thing so that the rollers on one side would crush the grain and on the other ride gently over the top. Not convinced that rolling is the best way of threshing, I would have thought it needs a different sort of mechanical treatment.
Anyway, this thing is spinning merrily and the wind is winnowing the grain from the stalks. It is also blowing the lightweight grain all over the place (primitive wheat species had much smaller grain size), which you then need to collect from inside and outside the circle and separate on the ground, this is known as winnowing........Oh....hang on.....
How do you feed a new supply of sheaves to the mill? With all those rollers going around I wouldn't fancy my finger's chances. Is this also the first example of the axled roller being invented before the wheel proper?

I have a few more objections but I will leave it to others to take up the baton.

A wonderful fun theory but in practice and evidence a bit of a non-starter
Jim.