The BBC has already announced that BBC One will become a full-time HD channel in the coming months, and that this will use the second "slot" that the BBC has on multiplex PSB3.
Half of PSB3 is shared by ITV and Channel 4 (S4C in Wales); the other half belongs to the BBC. The total capacity is 40Mbps, so the BBC has 20Mbps.
We know that the BBC is using about 8Mbps for each HD channel, and it is also known that MPEG-4 2Mbps provides a SD channel at we might call normal-Freeview-quality.
In addition, laudable as the BBC HD channel is for only showing HD content, this is not the way any other channel has gone. Following the lead of the US market, which has already switched to HD, channels have become "HD" with full-time high definition a future ambition.
In addition, BARB ratings, which the UK television industry uses, are much kinder to channels that simulcast in standard and high definition versions. The BBC HD channel's schedule makes marketing the programmes in HD an uphill task.
So, I expect that the BBC will launch the six main channels onto Freeview HD, so they can run in different configurations. So, for example, with BBC One HD showing HD alongside CBBC and CBeebies being off-air, the options would be:
And if BBC One had no HD content to broadcast (such as during regional news), the options would then be:
In the daytime (7am-7pm), BBC three and BBC FOUR would be off-air, so the channel capacity would be shared like this:
And again with BBC One showing SD:
By using some bandwidth to carry two SD channels, the BBC would have a more industry-standard HD service of part-time HD channels using the traditional brands and hours.
By mirroring this on satellite, the BBC would also get six channels into the Sky HD EPG.
And the BBC could say they have been saving money by "closing" a channel,