Hard to tell - but as that route is over 11K miles vs. around 8K via LHR (no idea if that is a viable route for OW though), you may have a bit of a problem.

But I suppose it depends how you can fly out of KHI on OW in the first place. If the only way is via BKK, your routing should be fine. I'm not surprised that ba.com has some difficulty with this one