This is the windmill that you can see in Spencer's first photo from Ivinghoe Beacon. It's a 'post' mill, so called because the whole tall top can be rotated around a central post, thus taking advantage of any wind from any direction. It was built in 1627, restored by volunteers, and is in the care of the National Trust.
Windmills and watermills are very ancient edifices; they served a whole village and would have been a prominent feature pre- Enclosure. I don't really know what happened to milling arrangements after the Agricultural Revolution, but it's not something that you would expect to find in every farm.
The picture is from Flicr:
And very pretty it is, too!