Not talking about box outputs, I'm talking about the broadcasts...
In the early Virgin HD days most broadcasts where in 720p, then they started going 1080i (for the very reason you mention) and as they where adding higher quality channels from Sky.
They where at one point broadcasting stuff like BBC HD in 720P... I remember clearly because I found out when compared Sky HD and was bitching at Virgin about it!
Now I'm not discussing it any further... the fact is I asked wether BT TV was broadcasting all its HD channels (or AMC at least) at 1080P HD or 1080i HD, this whole discussion has been a waste of time and still no closer to get an answer to the question originally asked.
and worse still it was all started by your jumping of bullets and not reading the question correctly or even attempting to understand its context!