Isn't that because the hardware used here doesn't support it?
If the SD and HD PIDs are on the same transponder, then it should work in the UK I believe. Sweden has been doing this for years to allow for regional opts on SVT1 AND SVT2 on satellite - but only in SD.
They have regional SIDs for all the SVT1 and SVT2 regional variations. Most of the time they point to the PIDS for the Network SVT1 and SVT2 audio and video streams, but when they opt-out on SVT1 OR SVT2 the regional SVT1/2 SIDs are re-pointed to the regional audio and video stream PIDS. This means they can opt-out on either SVT or SVT2 - but not at the same time.
However they squeeze all of the streams onto a single transponder to avoid a re-tune. And there is still a nasty transition (a grey screen for a second or so on the receiver I saw)
They don't do it for HD broadcasts. (And don't have regional news on their DVB-T2 HD variants either, and broadcast Stockholm's regional news nationally on the HD versions)