I have a new BT youview box but can't access the HD freeview channels. The 'Watch in HD' facility referred to in the guide (top 10 tips) does not show as an option when I am watching channels that are also available in HD.
The HD setting in the menus is for setting a preference for playback quality on the on-demand channels.
For watching live TV in HD IIRC the BBC channels sometimes have the option to 'watch this programme in HD' via the Info button menu. Similarly for recording. ITV & C4 don't do this (yet).
However to ensure that you watch in HD you should choose the HD channels directly using their channel numbers - i.e. '101' instead of '1' for BBC One HD, '102' instead of '2' for BBC HD (Soon to be BBC Two HD), '103' for ITV HD / STV HD / UTV HD and '104' for 4hd.