D'oh finished writing my reply only to see two people said what I have much quicker. Anyway I agree with them.
I think that's more to do with poor compression of HD over Virgin. As I said above, Virgin isn't fibre anyway. Cable is called cable, because it runs through coaxial copper cables. And a chain is only as strong as its weakest link so it doesn't really matter where they cut off fibre and run cable.
Anyway digital TV it doesn't matter about strength of signal, that's the whole point. With digital if you don't get the full signal, you lose chunks of data and you will see whole blocks of picture missing from the screen, or nothing at all. It's not like analogue days when signal strength affected signal quality progressively.
Sky (BSkyB), Freesat from BBC/ITV and the trial for terrestrial Freeview HD in London all used the H.264 video compression which is pretty neat. According to the internet Virgin HD is compressed mpeg2 from 1080i h264 sources - at a lower bitrate than the better compressed originals. Not sure how true that is but doesn't sound good.
Oh and also equipment quality plays a MASSIVE part in the quality too. Anyway lets just say after watching some decent stuff on Discovery HD and so on (wish I could get that on Freesat HD
) there is a noticeable difference with HD on some channels. The picture and also the sound, full Dolby 5.1 through some channels. I must admit BBC HD quality seems to be worse than before, but still noticeably better on the 40" LCD here compared with SD.