Doctor X

Doctor X was a television series created in the 1950s. The series had plots involving the Queen being an alien and monsters invading Westminster Abbey.

Matt Hansen was a huge fan of the show and liked to hunt for old episodes on YouTube. Andy Hansen and Erimem travelled back to 1964 to track down the first book ever published about the series, Doctor X in an Amazing Adventure in Space. The book was incredibly rare and occasionally sold for around 400 quid on eBay in 2015. (PROSE: In Search of Doctor X)

Giles owned a VHS of Doctor X, and on the spine, there was PG certificate and the BBV logo. (WC: Case File - Varunastra)

A Cyberon was once described as being something futuristic, like one of the monsters from Doctor X. (PROSE: Cyberon)

Behind the scenes
Like Professor X before it, Doctor X serves as an in-universe analogue to Doctor Who.

The storyline involving monsters invading Westminster Abbey is a reference to the climax of the 1953 science fiction serial. Doctor X in an Amazing Adventure in Space is a transparent reference to Doctor Who in an Exciting Adventure with the Daleks.