The A380 arm rests go fully up and you can lie down comfortably. I think the 787 arm rests go up about two-thirds of the way, so you can still lie down underneath them, but not as comfortably as the A380.

In my experience, QR seat maps typically look low until a few days before check-in (seats are not automatically assigned or blocked before that) but you may find that your LHR-DOH legs are quieter than the DOH-BKK legs. I often have a row to myself on the A380 LHR flights.

Have a great trip!