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. (PROSE: In Search of Doctor X) Rumour had it, that the company broadcasting the series ran out of money, causing bailiffs to take episodes of the series and throw them into landfill, hence making a number of episodes missing, leaving fans like Giles to search for any surviving copies. (WC: Case File - Doctor X)

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)

The Caverns of Andostar was a lost episode of Doctor X. (WC: Case File - Doctor X)

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, however, unlike the analogues that came before it, stories featuring Doctor X established clear differences, such as the premiere of the series being around a decade before 1963 and the cause of missing episodes to be entirely unrelated to the BBC's old junking system.

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.