Impartiality and being un-biased are different things.
The BBC is pretty much the gold standard for un-biased reporting, regularly taking the government to task on issues of the day.
It's only the "gold standard" for mainstream sources, which is not saying much. For people shopping for more accurate information elsewhere, it's nothing to write home about. Would you take your technical reporting from the BBC? Well, I wouldn't take my world affair's reporting either.
That said, I have worked with the BBC several times in the past (and other outlets, as a stringer). No mainstream coverage is accurate, and a lot are just used to justify a point the reporter has already made. Don't be fooled by spectacles like a BBC journalist "destroying" another BBC journalist on air.
It even takes itself to task hugely, as shown recently when a BBC reporter was destroyed in an interview about how much better the Sky and iTV coverage was than the BBC during the capture of Tripoli, by another BBC journalist live on air.