You can very easily go your bank at 9am deposit or withdraw say 1,000 then go in bank and ask them for one year statement and still get to immigration same morning or take a stroll and be on immigration directly after lunch no problem
If your a thai use thai passport to enter and leave Thailand
UK you can renter with UK passport
If you use a UK passport and your UK citizen or a Thai citizen and you use a UK passport to enter Thailand then regardless of nationality you are liable for visa