I read your post as I am also surprised that this is not possible. I am also surprised at the poor attitudes of the other two posters. They seem to take any criticism about Virgin as a personal insult.
I believe that this issue is an omission by Virgin that could easily be corrected by a software update. I have just moved from SKY and I am surprised that there is no simple default setting for HD channels.
I hope that you get better treatment in the forums in future!