Big Finish (in-universe)

Petronella Osgood had a folder labelled "Big Finish" on her computer. (WC: The Zygon Isolation)

Behind the scenes

 * In the real world, Big Finish is a company that has produced Doctor Who audio stories, as well as several spin-offs, since 1998.

Information from invalid sources
According to the Doctor's Five Hundred Year Diary, Big Finish was the "most successful creator to come out of Mutter's Spiral" - they were an Earth based production company associated with BBC's Doctor Who, and in 2021, they released the previously unproduced 1978 serial The Doomsday Contract by John Lloyd and Nev Fountain, starring Tom Baker, Lalla Ward, John Leeson, and Julian Wadham. (NOTVALID: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)

Big Finish (The Zygon Isolation)