KISS and use DVB-T
Stop the silly sell off of (all) the old analogue channels,
carry on using DVB-T, but to carry MPEG4,
use two multiplexes for HD.
MPEG4 should allow 2 or 3 HD streams per multiplex channel (on DVB-T), have one multiplex 16QAM for more reliable, but 2 HD streams (live coverage). And have one multiplex 64QAM for 3 HD streams of fluff (catchup).
DVB-T2 (according to Wikipedia) gives 30% more capacity (and have better error correction), so giving 4 HD streams per multiplex channel, instead of 3.