Awfully...Frightfully...
If you consider when this series was made, at the height of the era of so called kitchen sink dramas, when northern accents were fashionable, it is amazing how so many upper-class twits fill up the series. Made during swinging '60s London, but you wouldn't know it as there is no sign of a cockney accent in sight either - much less a northern one.
"CG is good. CG works" Gordon Ghekko