Andy Quested, Principal Technologist, BBC Future Media & Technology: "We don't do the encoding for the Virgin platform. The signal from the playout and continuity area goes straight to an inject point somewhere in the bottom of Television Centre and (I assume) off to Virgin's distribution centre. The Virgin distributed HD signal MPEG2 at around 17Mbs and has been for sometime. In fact I believe there has only been one encoder change since the test channel launched. MPEG2 is a mature codec so updates (hard and soft) don't come along very often."
So the BBC HD picture we get hasn't had its bitrate cut like the channel has on either of the satellite platforms, which means the degradation we're seeing is down to other factors. Interesting.
Incidentally, anyone relying on the HD section on BBC iPlayer catch up this Christmas is going to be sorely disappointed: it's missing most of the main Christmas shows (Doctor Who, Cranford, Day of the Triffids, Outnumbered etc). They're there in standard def, but no HD.