What old equipment can do ...
AC, @13:13 on the 17th isn't quite correct.
The previous transmissions were using AVC/H.264 as well - there have been no HD MPEG2 transmissions in the UK (though MPEG2 in HD has been used on some cable services).
The first round of tests from Crystal Palace used DVB-T and H.264, so there's nothing new in the use of that codec from next month.
What is new is DVB-T2, which increases the carrying capacity of a mux by around 50%, and will be used by some other countries in Europe as they move to HD as well - we'll simply be the first, regaining the lead we had when we launched digital terrestrial.
Yes, some existing HD Ready sets would be able to receive HD is DVB-T2 weren't used, but not all of them - it's only in the last year or two that they've routinely had decoders capable of receiving H.264; even if MPEG2 were used (which would be hopelessly inefficient and mean switching off other channels to fit in just one HD service), plenty of sets wouldn't be able to receive the service anyway - if they're designed with an SD decoder (which many have been, in the absence of broadcast HD) - then you're still stuck with an MPEG decoder that can't create a picture bigger than its 720x576 frame buffer.
So, whatever was done, lots of old equipment, regardless of the resolution of the screen, would not cope with HD.
Long term, as people want to screw more money out of the spectrum, there'll be a move to H.264, to fit more channels in. So that makes sense - especially for HD. And for the same reason, there'll be eventual pressure to move to DVB-T2.
The UK plan recognised that telling people to buy a set now with H.264 for HD, and then upgrading again in a few years for more capacity, with DVB-T2 wouldn't be popular. So the decision was taken to link together the move to both technologies.
At the cost of a later start to HD than some other countries, and an initial scarcity of equipment, it at least means that for this change you'll get both technologies in one go, instead of buying an HDTV receiver now, and finding that they decide to change the modulation system in a few years, necessitating another purchase.