Upminster Windmill
The windmill at Upminster, to the East of London, is preserved in working condition in a park on a hill close to the railway station. Whilst the windmill can grind wheat into flower, they don’t (as of 2023) because the mill has not been conformed to food safety regulations.
I also have photos of Shirley Windmill and Brixton Windmill on this website.
From the top of the mill, on a clear day, the Crystal Palace transmitter can be seen over 16 miles away.