British TV tends to make TV series of 6 or 12 episodes at a time, rather than the US norm of 26 or 28. Doctor Who is made in series of 13 episodes, and most comedy shows like the Young Ones and Bottom are made in batches of six. The thinking tends to be that the series will end up with less dud episodes; quality rather than quantity. Far from being cancelled, Fawlty Towers was brought back three years after the original run for another series after Cleese and Booth were persuaded to do another.
https://en.wikipedia.org/wiki/Talk:Fawlty_Towers#12_Episodes.3F