You can't receive HDTV through your aerial. HDTV requires a digital signal but there's not enough bandwidth to provide HDTV with Freeview.
The BBC is currently broadcasting a few hours of HDTV per day. These programmes are subscription-free and available on Freesat. (If you've not already got satellite TV, Freesat involves a one-off payment of �150). Otherwise you need to subscribe to one of the commercial HDTV services. These are available on satellite from Sky or on cable with Telewest.
Source:
http://www.bbc.co.uk/digital/tv/tv_hd.shtml
Chris