So are you saying that there won't be space for many HD channels via Freeview in the future? Not good and surely contradictory to the TV companies desires?
When Bands IV and V were allocated to 625 line TV the broadcaster engineers carefully selected the channel allocations to avoid interference between adjacent transmitters and they managed to fit four national analogue networks into the available space. Since then Channel 5 has been shoehorned in, mainly using channels around 36/38 that were left free for other users (radar and radio astronomy then later used for VCR modulators), but it's coverage is poor in many areas. If they had to, they could probably have allocated the 48 available channels (21 to 68) to more than four networks from the start, but it would have been at the risk of increased interference in fringe areas. It's not just channel clashes in overlapping service areas (causing co-channel interference) that have to be considered, there are other types of interference too, such as adjacent channel.
Now they have managed to fit in 6 multiplexes in addition to the existing 4 or 5 analogue channels, it seems possible that the allocation rules can be relaxed for digital TV. However the mulitiplexes are on low power at the moment to avoid interfering with the analogue transmissions, and only being transmitted by the main transmitters, so the service areas are smaller with much less overlap, so there is less need to worry about the effect of adjacent transmitters.
This will change when analogue is switched off and the digital transmitter powers are increased, making the service areas larger. I suspect they will have to reallocate all the multiplexes from scratch to reduce interference from adjacent transmitters. I haven't seen any mention of this, but that's what I suspect may happen. And that's probably when the government will step in and limit the spectrum available for the reallocation.
I suspect the allocation rules could actually be relaxed a bit for digital, because the effects of interference on digital are different from analogue. It may be possible to fit in, say, eight national multiplexes (don't quote me, I plucked this figure out of thin air) provided the current 48 channels are retained. If that were possible then there would be two extra multiplexes available nationally. They could be used for extra channels and/or to improve the quality of the existing channels. If the government pinches a chunk of the spectrum to sell off to other users then the broadcasters might only be able to fit the existing six multiplexes in the remaining space.
So where does HD fit in? With current compression techniques I think 2 or 3 HD channels can fit in a multiplex, depending on the type of compression and whether 16 or 64 QAM is used. So if we were to finish up with eight national muliplexes, two of these could be allocated to HD, giving a total of 4 to 6 HD channels. That's based on my guess of eight multiplexes, I have no idea how many we will actually finish up with. As compression techniques improve it might be possible to fit more HD channels into a multiplex. In practise, the HD channels would probably be multiplexed with SD channels and spread across the multiplexes. But it seems quite possible that several channels could be transmitted in HD - provided the government doesn't pinch the spectrum they would need.
Different people will have different opionions on how the available space in the multiplexes should be allocated. Some people must watch what I consider to be rubbish channels otherwise they wouldn't survive. I would prefer quality rather than quantity. I rarely watch much outside the main four/five channels anyway, mainly because (a) I prefer programmes made in this country and (b) I can't be bothered to search through all the channel listings to find out what's on. So I would be quite happy if only BBC1, BBC2, ITV, Channel 4 and Channel 5 were available in HD, together with perhaps a sports channel and a film channel.
Sorry, this is a long reply to a short question. I think the main message has to be that the government could severely limit the development of terrestrial TV if they go ahead with their plans to sell part of the spectrum it uses and needs.