Flat Holm

Flat Holm was an island located south-southeast of Cardiff Bay in the middle of the Bristol Channel.

History
Assumed to be uninhabited scrubland, the island was secretly used by Jack Harkness to care for and keep safe those taken in by the Rift and returned, scarred mentally (and sometimes physically). When he took over Torchwood Three in 2000, Jack found two survivors in the vaults. He created a place where they could be cared for, calling them experiments which had gone wrong to the carers. Jonah Bevan ended up there, as Gwen Cooper discovered. Jack had kept the facility secret from her. (TV: Adrift)

Gwen left Simon Peters at the Flat Holm facility, taking a retcon pill to forget about what he did. (PROSE: We All Go Through)

Alternate timeline
In an alternate timeline created by John Hart, Queen Victoria, using powers granted by the Extraordinary Circumstances Act of 1879, sentenced Jack Harkness, who John had framed as an assassin, to perpetual confinement on Flat Holm. (AUDIO: The Death of Captain Jack)