I am trying to remember the name of a miniseries I watched as a child from the 1990s, probably produced by the BBC. It had plots within plots and episodes reveal that some of what is taking place is literally inside the main character's brain. I think there were only about 3 episodes.
This is The Singing Detective, a 1986 BBC miniseries by Dennis Potter. It tells the story of mystery writer Philip Marlow, hospitalised with psoratic arthritis. In hospital he hallucinates an escapist 'film noir' fantasy world with a private detective, also called Philip Marlow, as well as flashbacks to his English childhood.
In Karaoke, the lines between fiction and reality become somewhat blurred for the protagonist, Daniel Feeld, a playwright. He discovers he is dying and leaves his body to cryogenic research.
In Cold Lazarus: following his death, his disembodied head is kept alive in a dystopian future, where researchers electrically stimulate it to extract various vicarious pleasures from his memory for broadcast.