@Tom ******
the required 800,000 THB is a tad more than just 18,000 GBP. I suggest you place 19,000 GBP into your British account, and for the application in Thailand for the 1-year extension, just have 850,000 THB in your Thai bank account, for your own safety. Once you got this deposit, you can't get under it anymore due to exchange rate fluctutions