At Heathrow Terminal 5 everyone goes through the same channel. However, I believe that photo-ID is required of domestic travellers, not a passport, and domestic travellers aren't subjected to "immigration" checks (using "immigration" in the sense of checks by the immigration authorities). The point is that immigration checks for the departing passengers has been coalesced into one with security checks, but that doesn't mean that the two processes are the same.

I have heard stories of some airlines (though not I think BA) seeming to require domestic travellers to present a passport as the acceptable proof of identity at check-in, although I don't know if that is true: in other words, would something like a photo driving licence be acceptable to those airlines?