Write to the British Consulate in Bangkok with (copies of) evidence of your total annual income - which should add up to ฿800,000. Within two weeks, they will send you an income letter that you present to immigration. (If you do it today and explain that you need the letter back urgently then they may be able to send it in time.) No need to muck about with cash in bank that needs to be "seasoned". The cost is £52.
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