Posh hasn't shared the stage with her fellow Spices since 2012, sitting out their most recent tour.
The former Posh Spice tells British Vogue that singing was never her "passion".
Last year's festival was cancelled due to the coronavirus pandemic.
The singer-turned-fashionista has explained her behaviour in the decades-old footage.
The Spice Girls star has dusted off her acting skills to play Elizabeth I, who she's hailed as a "Queen of Girl Power".