Not that I'm such an LHR expert, but I do recall seeing flights on the monitor with "will depart from Terminal B" notations long before the actual gate is posted. People on those flights can then move over to T5B and use the lounge, if qualified.

Another trick: When I go through Flight Conformance, I ask the attendant if the gate for my flight is "penciled in". I use that phrase to make sure they understand I know it's tentative and to still watch the monitors. If I know the "penciled in" gate, I can go into the departure area and wait near that gate, thus saving myself some time when the gate is announced and it's time to move.