Quite legal for an immigration office to hold on to your passport and give you it back the next afternoon with the visa in it as done it in Phuket many times ,agency's are another question as I would never trust some of them ,heard too much horror stories
Never put 8.4 million baht in one bank in Thailand,interest is less than 1.5% ,and most banks only insure your money up to 1 million baht ,keep it in UK bank or invest it and take small lump sums at 6 month periods to transfer into a Thai bank