Why isn't Doctor Who broadcast in High Definition?

In response to a viewer question on why the BBC chose to broadcast Torchwood in high definition but not Doctor Who on the BBC opinion programme Points of View, producer Phil Collinson explained:"Torchwood is shot in HD, and Doctor Who isn't. Quite simply, Torchwood has about a third the number of special effects shots that Doctor Who has. In that way its a much simpler series and so because they have less special effects its going to take less time to make those special effects."So the simple answer is we wouldn't be able to deliver a series of Doctor Who in high definition every year. It would probably take us twice as long to make so therefore viewers wouldn't get a series every year, they'd get a new series every 18 months."Later in the programme, Mary Fitzpatrick, Editorial Executive for Diversity, responded to another viewer's question on the lack of prominent black people in BBC programmes, citing the Doctor's new assistant as played by Freema Agyeman as an example of how such actors are appearing in popular shows.Presenter Terry Wogan also commented that Doctor Who is the most discussed series on the Points of View messageboard on the BBC website!