Dave, KMJ: Channel 5 withdrew due to "financial constraints". It is widely believed that those financial constraints include some incentive from Sky for the channel to remain exclusive to pay services. That may be as simple as Sky paying the carriage costs on satellite, since the satellite transponder is shared with Sky Sports News HD and ESPN America HD. It may be that Channel 5 thought they could renegotiate this contract or break it early, but found that the compensation to Sky was too high to be covered by the expected ad revenue from Freeview. Which, to be honest, is unlikely to be substantially greater than the current revenue, as many viewers will simply switch from watching C5 in SD to watching it in HD.
On top of that, Channel 5 SD services have regional advertising - there are five versions of Channel 5 SD on satellite for this reason.
If Channel 5 wanted the HD service to go free-to-air on satellite, they would first have to lease a transponder, or space on a transponder, that uses the UK spot beam (although the current transponder is on the Astra 1N satellite, it uses the pan-European beam). Channel 4 and the BBC have spare capacity on the transponders they use for their HD services at present.
That's not a pre-requisite for going on Freeview, of course: as others have said, the UKTV stable are on Freeview without subscription, but require a subscription on other platforms.
I'm curious to know why the band-plan graphic above has gaps and why there is no channel 26 at all!
Even if the frequency ranges covered by the gaps are currently assigned to other services for the foreseeable future, surely it would have made sense to use a contiguous sequential numbering system across the entire band.
Where is that elusive channel 26?
Also, when new frequencies are allocated to DVB-T2, will set-top-boxes and TVs automatically recognise them or will we be looking at hardware upgrades?