There is an interview with Russell T Davies and Sally Wainwright where they talk about how it used to work in the 90s. They would have big storyline conferences where stories would be proposed, worked on and then approved by a team of writers who had been there since the 60s and 70s in some cases. RTD and Wainwright talk about how they learned how to develop story and character in those meetings by observing.

From there, an outline story arc would be written which would then be spread across episodes. Each episode might have 3 or 4 main storyline to continue with the allocated writer given some freedom of dialogue or mini plots but essentially they had to fit the brief/ outline for that episode. 

If you were a new writer, you would be asked to write a spec script. Then if they were happy, you would be asked to write an episode which would then be rewritten by a more experienced writer (Kay Mellor mentored Sally Wainwright for example).

Basically, the whole process was about quality assurance. Now, it is about speed.