There is an oddity with the redemption booking engine, it does not see the SYD services via BKK, but if you look for LHR-BKK and then BKK-SYD (or vv.) you get to see availability.

I have a F redemption out via SIN in Jan and back in Feb via BKK and when I was looking at option dates for the return I had a choice of SIN or BKK, I went for BKK as I preferred the 747.