bagold,
if you originate in Asia you are permitted 2 stopovers and they MUST be at your intercontinental gateways. You are permitted 5 segments within Asia and you may do any routing you chose including connecting through Hong Kong more than once.
Therefore if you want to do HKG-BKK-HKG-LHR you will not be permitted to stop in BKK since HKG is you intercontinental gateway. That said the prices out of BKK are signifigantly cheaper then prices out of HKG, therefore buy a seperate ticket from HKG to BKK and originate your RTW in BKK with a stop in HKG before your flight to LHR and you will accomplish what i believe you want at a lower price.
rich