Well, Doctor Who and Monty Python <em>are </em>a once-in-a-lifetime experience! If you like Monty Python, try Fawlty Towers or Ripping Yarns next. Personally, I found catching up on all the years I missed of Doctor Who worth it (mostly because I need to get a life), but the new series is self-containing, so you only need to start at 2005. And speaking of modern stuff, Merlin, Sherlock, Psychovilles, and Misfits are not half bad, either.

And if you do not mind non-fiction, the Poms have produced some of the greatest documentaries out there: Civilisation, the Ascent of Man, the World at War (I & II), anything by David Attenborough, anything from the 'Walking with ...' series.

 Have fun!

My favourite British TV series is Peep Show. It's an interesting comedy.

I can name you some quality British TV series.

First I highly recommend Monty Phyton, Little Britain (includes different biritsh accents), Doctor Who (my favorite) and last but not least Sherlock. :)

I suggest you to listen to podcsts if you want to improve your listening. There are tons of podcasts at BBC site you can download them as mp3 files. Most of them have transcript and it is very useful. 

Coronation Street is a good series to watch.