Mr Erko,

I guess the "problem" will be on the SYD-BKK-sectors where capacity/quotas are very limited due to the fact that TG offers only one flight per day.
May I recommend you to have it booked "step-by-step", not in one single step like SYD-LON-SYD but have it done SYD-BKK, then BKK-LHR, then LHR-BKK and finally BKK-SYD.
You will definitely need personal assistance by your local TG office which is overlooking your membership activities - they might be able to help you with your request....