There isn't much difference. Both B and D are going to be bus gates for the Embraers. B is a side area within A where the buses can leave from, and D is a separate but connected building. BA's service to LHR have left from gate A5 for as long as I can remember, it's almost as if BA have a lease on that gate. Gate B departures are next to G0 (only airport I can think off with a zero gate, TXL is good at being slightly weird) on the other side of the circle, so it's actually very near the BA lounge which is between A0 and A15, again on the circle design. The BA lounge is alongside Starbucks (you'll see that sign first) slightly towards A15. So given a choice B is nearer than D and less prone to a delay at security than D, which can get a bit congested if there are multiple flights.