I would also look at how much cheaper it would be to originate out of BKK on a OW RTW in F. It might be worth getting a cheap RT ticket HKG-BKK on another airline to start your trip and avoiding CX all together during these rocky times.

BKK-LHR on BA/QF
LHR-MIA on BA/AA
The return trip from MIA to BKK could give you a number of possibilities depending on the time you had and how many miles you would want to earn.

Most direct (3 continents)
MIA-SJC-TPE AA TPE-BKK BA
or
MIA-DFW/ORD/SEA/SJC-NRT NRT-BKK BA?/JL (AA codeshare if you can do this on OW RTW?)
or
MIA-DFW-KIX AA
KIX-BKK (? - don't know if any OW airlines will work for this on a direct flight)