Well, TV in the UK isn't like TV here. Over here, by the time a show gets to it's 100th episode or its 3rd or 4th season, it's just getting going. In the UK, though, it's pretty commonplace for a show to have only two or three "series" of about 6 or 7 episodes, and then that'll be the end of it. Take "The Office", for instance. Thanks to the US version, it's one of the best known British shows over here (even if people haven't seen it, they still know about it), but it still only has 2 series of about 7 episodes each, along with 2 Christmas specials.

Doctor Who has always been an exception to the rule though. It ran from 1963 to 1989, then there was a TV movie in 1996, and then they revived the show in 2005. I guess really it'll be something like season 28 or something like that, but it's labeled Series 4 of the "new" series.