I wonder if the issue is the 2-4-1 voucher and whether it can only be used for a simple XXX-XXX return (which in your case was LHR-SYD which is deemed a direct flight despite the refuelling stop). By changing flight in SIN or having a surface sector you are now looking to go LHR-SYD-SIN-surface-BKK-LHR which is beyond what they think is allowed on a 2-4-1?
I had a quick look at the T&Cs and can't however see anything to that effect other than:
"All Companion Voucher bookings must originate and return to the UK." to which BA may argue that your flight BA16 SYD-SIN neither originates or returns to the UK.