Thanks for the clarification. That makes sense. I would really like to do HKG-BKK-HKG-LHR (day trip to BKK) but it sounds like I can only do HKG-BKK-LHR.
If I do the latter routing, do you know whether they have showers at the lounge in BKK? Would really love to have one before going to LHR after being out in BKK for the day.