The OP has learnt a lesson from this but must, unfortunately “take it on the chin”. We’ve all been there at some stage and also learnt that lesson. Some of us have fallen for it a couple of times.

The OP has also discovered that the booked flight can be rescheduled for a THB 6.600 fee, a little less than $200, which is a relative drop in the ocean compared to the original ticket cost. I think in the grand scheme of things this can be seen as a result.

Perhaps the BA agent the OP contacted could have been so friendly to point that out, but unfortunately that only happens with well trained and empathetic agents, which are a rapidly disappearing breed at BA.