SIN is a lot closer to KUL than BKK, so why not do a KUL-SIN return, either on Avios, or on Air Asia for about £65 return.
OR fly back from SIN to AMS and do the short-haul trip from SIN-KUL. That would enable you to fly Cathay from BKK to SIN (wide body, lie-flat, as opposed to Malaysian's 737s)