...the maps made by the Polish equivalent of OS use two symbols for windmills - one with sails, one without. I don't think they do that over here.
Not since about 1974. The OS Seventh Series immediately preceded the 1:50000 series (becoming known as Landranger in 1979), and included these symbols:
but while the 1:50000 uses symbols for wind pumps and wind turbines, there is a single symbol for windmills, whether they're active or disused, or derelict, like
Sutton towermill that I visited when I was very young.
I went to see Holgate Mill in York, back in the summer, and since then it
has its sails again and looks quite splendid.