You might want to look into CX's Agent fares ex-BKK, BKK-HKG-LHR v.v. runs to just over £1100 in I Class (full tier points), and the return can be ticketed as open. Stopovers are allowed in HK on the LHR-HKG-BKK portion only. Ticket is fully changeable AFAIK.
Regards,
Alex