The secret of British TV screenwriting

I have grown up watching British classics like "Are you being served", "Keeping Up Appearances", "Father Ted" and "Yes Minister". I have also stuck almost my whole life with "Doctor Who" and I am now watching "I, Claudius", where every single scene feels in place and well thought out and one memorable line of dialogue follows another.

I cannot escape the conclusion that the writing of ye olde British TV is of excellent quality, has a very consistent feel to it and frankly seems to be a whole league above modern series. I may sound like I am old and whiney, but I found some screenwriting cues from on of the current industry giant the other day and it really seemed to me to be something no one would take seriously in, say, BBC in the times of yore.

I never did much research on the screenwriters and maybe I should. Was there a "school" of TV writing in Britain in, say, 1970 - 1990s? Was there some mentor of a handful of them? What books would these people be reading? Did they come up with some core of screenwriting rules or such?