The "advanced service" is on unknown frequencies, so it is possible that there might be different coverage to standardFreeview and a widebandaerial might be required. The advanced services will move to their correct frequencies at switch-over.
Gavin: Should your model of TV offer a manual tuning facility then enter and scanC57 which is Oxfords HDchannel, if the situation remains the same after having carried the aforementioned out then could you please provide the model number of the device in question.
By the way, when you say that the BT Freeview box can receive HD when plugged into the same aerial, does this refer to viewing an HD channel on the box "immediately" before tuning in your new TV? because if any gap exists between the two then the HD signal may have dropped out due the widespread problems being caused by high pressure interference