If you must use BA (for the sake of a GUF etc), then you can go ex-AMS on the widely publicised fare:
https://www.google.co.uk/flights/#se...=ONEWORLD;eo=e
But go via HKG at both ends at the BA product to BKK is disastorous.
My preference would be the QR offer from OSL mentioned by @
Woody12354 above as it is both cheaper, and much better quality product.