Royal Mail has issued a Windmills and Watermills special stamp. The “first day cover” features Brixton Windmill and six special edition stamps. They can be bought in an envelope printed with an image of Brixton Windmill taken by Bugle photographer David Wilcock. You can pick them up at the windmill open days, organised by Friends of Windmill Gardens, The next open days are: Saturday and Sunday 8 and 9 July and Saturday and Sunday 12 and 13 August. More information here.
Brixton Windmill, as the headline suggests, is unfortunately not on one of these stamps. It is on the first day cover envelope, which is an entirely different thing. These envelopes are all different depending on or what organisation prints and issues them. Brixton Windmill is also on the franking mark but again these are different across the country.