The UK’s closest equivalents to the US showrunners are Steven Moffat and Armando Iannucci. Few others are allowed to go off and wrangle a team of writers, with only the lightest of touches from the higher echelons. Even Moffat had to re-pilot Sherlock, although the fact that the BBC allowed him to do this shows the respect and faith they (rightly) accord him. I’m on a far lower rung, and so for me, part of the appeal of novel writing is that the chain from author to reader is short and simple – agent, editor, proofreader, shop/website.
In TV, the script will have to be signed off by producers, executive producers, genre commissioners and channel commissioners, and that’s still only a starting point; the director and actors (once they’ve been approved) will then have their say. None of this is necessarily bad; any writer will welcome informed opinions that improve their work. But the longer the chain of opinions that have to be taken into account, the more the danger of weak links. Hence the (possibly apocryphal) tale of the executive whose main note on the script of The Manchester Passion was “more jeopardy for Jesus?”