I have just done a similar booking and a similar calculation - I just could not justify using BA when QR to BKK was far better value. It also makes me question whether I will keep the BA Premium Card now - the US redemptions are just stupid pricing going West and QR are far better value (and product) going East.
FRA can be a good starting point for QR too. However even starting in LHR I couldn't justify the increase.
aks120