Virgin Media started using some wonderful new encoders at the end of last year that greatly reduced the amount of space needed for HD channels and has enabled tham to put up to 14 (IIRC) HD streams on one multiplex.
It seems this has arrived on Satellite. It may be it has been present for a while, but I only just saw a post on Digitial Spy from omnidirectional that pointed to what seems to be the same thing on a satellite transponder. The thread was asking about DVB-S and DVB-S2 carrying HD but this was mentioned.
This transponder is DVB-S and is carrying both HD and SD channels.
The evidence of the new encoders is that the bitrates for MTV HD are roughly the same and often a little lower (!) than the SD version. And though Nick Jr SD and Comedy Central SD aren't on this transponder for direct comparison, the Nick Jr HD and Comedy Central HD streams are similarly low bitrates. Just look at the other SD channels, they are in similar Min/Avg/Max range to the HD channels. And this is on a DVB-S transponder, the old tech which allows less bandwidth per transponder.
This bodes well for increased HD, finally, on all platforms.
I'd imagine this is what the BBC is intending implementing to achieve BBC1 regions to be all HD by the end of 2022. No extra transponder space may be needed. There was a story recently (from the satellite company) that the BBC renewed their deal for satellite space, and I seem to recall it said the deal was for the same number of transponders, or implied as much.