I'm with Duggeh on the 6 episodes per series
e.g.
Fawlty Towers - Wikipedia, the free encyclopedia Twelve episodes were produced (two series each of six episodes).
Father Ted - Wikipedia, the free encyclopedia The first series comprised 6 episodes
The IT Crowd - Wikipedia, the free encyclopedia and has run for four series of 6 episodes
I heard it put down to the fact that the UK shows typically have the original author(s) as writer(s), whereas US shows employ teams of writers, with the end goal being syndication for multiple 13 episode series ....