Welcome to Flyertalk kennypanthew, and welcome to the BA forum, you have come to the right place to make the most of your Avios and using BAEC.

Because you're shifting from BKK to SIN I can see why that can't be done, it's a different zone presumably. I must admit I was under the impression that the booking could be truncated to be just a single trip to BKK and the return Avios thereby refunded (and the cash component, after paying the redeposit fee), and I hope someone else can confirm that understanding (or not). Having said that, if there is no cost disadvantage, it is often a good idea to make Avios bookings as single trips to preserve flexibility. The previous problem there was 2-4-1 vouchers but it turns out these can be split over 2 bookings too.