What it boils down to is that if you use the return fare at some point, so delaying the China sector until much later, then you will only have that change fee (£150), plus any fare difference, particularly if the BKK-NCL sector is flown already when you make the change. If you don't fly that return to China at all it may not result in much of a refund.

I would consider booking BKK-DOH-EDI, since Qatar do reasonable single fares. We have way too many external variables here too make a sensible choice.