The cheapest miles route would be Zone 2 to Zone 8 (HKG) which is only 37,500 miles + £255 in J. However, that assumes your parents are happy doing LHR - CPH - FRA - HKG or a similar routing - otherwise its 52,500 miles + £360 each from Zone 1.

There is no saving from starting in Zone 2 if you book a ticket to Thailand (Zone 7) - its going to be 60,000 miles + £405 each in J from Zone 1 or Zone 2.

Not sure if you could route to Thailand via HKG. Do a search of the 'Please post your successful redemptions' thread and see what other people have managed. However, internal Asian flights are usually very cheap, whether you use Air Asia or one of the mainstream carriers.