Because it's a QR ticket on to a QR flight my view is that it's best to go to T4. Not least because at the moment you have a chance of making the 21:55, just about, though it would require the HAL bus service to T4 to be fairly prompt. As things stand you have some chance to get to T5 for 20:30 hrs. The BA app won't do a robotic on a QR ticket, so it's over the QR's processes really. You would be allowed on the bus, there's no boarding pass check there.