According to Wikipedia between 2005 and 2008 Doctor Who, one of the BBC's flagship programs, was shot in 576i (standard definition). However, Torchwood, a spin off of the popular franchise which ran during a similar time period, was shot in 1080i (Full HD). Both series were produced by the same company - BBC Wales.
What explains the difference in quality? It's evident that the technology was there and that the production studio was willing to use it, so why didn't they?