Sky HD and Channel 4 HD are both quite heavily compressed.
Channel 4 HD using grass valley encoder which is believe use H.264 as the video compression standard, or selection of standards from the MPEG4 framework. I'm not sure if they have started using Dolby 5.1 yet.
the BBC HD channels use the same technology, with BBC HD on dsat handling the 5.1 slightly different to HD terrestrial which uses a slightly different method of 5.1 encoding.
ITV HD is probably the worst out of the bunch. It's mostly upscaled and the 5.1 operation is a bit wacky.
I've seen uncompressed 1080i HD from a few different places, and its much better than what is transmitted. Transmission is still better looking than MPEG2 video at roughly 5Mbps. When it comes to Interlaced vs Progressive scanning, each has its own benefits in relation to the content you want to show... it's not a case of Interlaced scan is the worst at showing everything. I've also been to conferences where 720p, 1080i, 1080p sources where being compared by industry professionals on decent large flat panels (not domestic gear) and looking at the run right next to each other, you can see a difference depending on the source more than anything else. Its ok for blu ray and things like that but comparing that to your TV signals is a bit different. especially when it comes to transmitting 1080p, which is hard. wouldn't surprise if people move to 1080psf and call it 1080p.
