The TR OP just checked the seat map a couple of days before the flight and discovered by chance that his flights showed a QS seat map (the LHR flights are actually scheduled QS on Wednesdays now). So this was just luck and can't be replicated that easily. The only way to do this is by keeping an eye on the seatmap and to book at very short notice.
If the OP would have flown a day or a week later he wouldn't have gotten QS seats.