Originally Posted by
PTahCha
It's because BA and VS only use one terminal at their homebase in LHR and elsewhere, whereas UA uses both Terminal A and Terminal C at EWR, as an example. The flight is assigned to the terminal/gate the day of the flight, hence why they can't provide the information upon booking.
BA actually uses 2 terminals at LHR - T5 and T3. And while they may tell you one terminal at booking, it can actually change if they decide to change which flights are using Terminal 3 (happens more frequently than it used to). So the OP would be best to not rely on what it says at booking when booking months in advance, no matter the airline.