I agree with what Ed Vaizey said in 2014 about a move to DVB-T2 and MPEG4 (at least for the PSB channels) along with the closure of the PSB SD channels.
https://www.gov.uk/government/speech...up-summir-2014
This should have been carried out before the clearance of 700 MHz got underway. I think they were afraid of upsetting the electorate as some would need a new STB, but i'd hazard a guess that most people now have a compatible box and the new ones selling today are only about £20.
Channel 4 +1 HD and 4Seven HD (along with other SD channels in DVB-T2) were created (by using the old On/ITV Digital infrastructure) to encourage take up, which seems to have worked.