Adventure with a premise strange enough to sustain itself
Science fiction and fantasy television earns its place in a schedule by proposing a world, or a rule, strange enough to generate stories indefinitely: a machine that travels through time and space, a spaceship crewed by escaped prisoners, a village that will not let anyone leave. Once the premise is established, the format runs like any other serial adventure — a recurring cast, a changing set of dangers, a season built from multi-part stories rather than one continuous film.
Studio constraints shaped the genre
Early British television science fiction was made on tight studio schedules, largely recorded close to continuously with videotape, sets built for a fixed number of cameras and effects achieved practically rather than digitally. That constraint shows up on screen as a particular look — solid sets, deliberate pacing, tension built through dialogue and situation rather than spectacle — and it's part of what gives the genre's classic period its distinct feel compared with the effects-driven science fiction that followed once production techniques changed.
The shows that defined the format
Quatermass put a scientist at the centre of a series of serials confronting threats that were alien in origin but investigated with the procedures of a crime drama, and its influence on how British television approached science fiction as a genre worth taking seriously is hard to overstate. Doctor Who took the format furthest: a time traveller whose ability to change appearance and character let the series recast its lead without ending the story, giving it a structural trick few other formats have ever had available to them, and letting the series move between historical adventure, pure science fiction and outright horror within the same run. Blake's 7 inverted the usual heroism of the genre, following a crew of escaped convicts fighting a totalitarian government from the margins, with a moral seriousness and a willingness to let its characters fail that set it apart from more straightforwardly heroic adventure series. The Prisoner sat at the edge of the genre entirely, following a man trapped in an ambiguous, surveillance-heavy village and never fully explaining why, using the trappings of spy fiction and science fiction to build something closer to allegory than adventure.
Tone varies more than the premise suggests
Don't assume all four sit in the same register. Doctor Who moves deliberately between comedy, adventure and horror depending on the story; Blake's 7 stays committed to moral ambiguity and rarely lets its heroes win cleanly; Quatermass is closer to a slow-building thriller than an adventure serial; The Prisoner resists straightforward interpretation altogether and rewards viewers who don't need every question answered.
Starting points and access
If you want adventure with room to breathe, start with an early Doctor Who serial rather than a later one, since the format's pacing and studio-bound style are most visible there. If you'd rather see the genre used for something harder-edged, Blake's 7 or Quatermass will suit you better. The Prisoner works best watched as a complete run rather than dipped into, since its individual episodes lean on each other more than a typical adventure serial. As with other archive television, availability depends on what a broadcaster or library service has chosen to preserve and reissue, so treat any single source as a starting point for your own search.