The BBC has always claimed to favour impartiality, both on the grounds that impartiality of the broadcasting media is important to a democratic society, and on the grounds that overt or mendacious propaganda is not effective in the medium term, since trust in media is eroded.
The meaning of "impartiality" needs to be studied: many commentators consider that the BBC has not been impartial, or even that impartiality is not possible.
This argument has exploded into public consciousness several times. The most well-known example is perhaps when a documentary programme ("Maggie's Militant Tendency") criticized extremism in the Conservative party. Here is a documentary about a slightly less well-known case - the 1971 documentary "Yesterday's men". The Labour Party had lost the election of 1970, and this documentary interviews Labour leaders. The title of the programme shows opposition to the Labour Party, and the content of the interviews was allegedly biassed against Labour.
This article from the BBC history website shows that the BBC today does not defend the programme.
Was it an unfortunate exception, or a sign that the BBC are in fact partial? Opinions are divided.