Gytha Ogg (usually called Nanny Ogg) is a character, a witch to be precise, from Terry Pratchett's series: Discworld. The 1st class stamp featuring Nanny Ogg forms part of an issue called "Magical Realms", released on March 8, 2011. The stamps feature characters from Magical Adventures including Harry Potter, Discworld, Narnia and Arthurian Legend.
An image from We Will Rock You appears on the 1st class stamp on the far right. British stage musicals have in fact been celebrated with eight commemorative stamps issued on February 24 last by the Royal Mail.
Thanks Dean for your nice mail! :)