LONDON — Few, if any, British TV toppers have ever enjoyed such enormous power over scripted shows as Jane Tranter, following her promotion to the newly created post of controller of BBC fiction in September.“Her influence and importance are huge,” says Andy Harries, Granada’s head of drama and comedy, and one of her chief rivals. “It is hard to know what Jane will do next because no other job in British television comes close in terms of editorial influence.” She is undoubtedly the most successful creative plying their trade at Television Center, the pubcaster’s West London headquarters. Tranter, 44, now runs an empire that oversees drama, comedy acquisitions and BBC Film, and extends across all four of the BBC’s main TV networks, BBC1, 2, 3 and 4 — the latter feted for its cost-effective, well executed one-off dramas. If she is as successful in her new role as she was running the BBC’s drama department, which she modernized following years of underachievement, the pubcaster’s competitors are in for a tougher ride. Astute politically, Tranter is known to favor working with a close circle of, usually, female lieutenants. Julie Gardner, head of drama at BBC Wales (where she helped mastermind the revived “Dr. Who” and spinoff skein “Torchwood”) is one such exec. “Torchwood,” pitched at an older audience than “Dr. Who’s,” broke all viewing records for a U.K. digital channel, attracting 2.8 million viewers and a 13% audience share for niche web BBC3. Following last winter’s multiaward-winning reinvention of “Bleak House,” Sandy Welch’s adaptation of “Jane Eyre” (BBC1′s big period piece this fall) was another triumph, albeit on a more modest scale.