How can I visit Thailand for 90 days with a UK passport and a potential Thai passport issue?

To stay in Thailand for 90 days as a UK passport holder, you initially need to obtain a 60-day tourist visa before your trip and then apply for a 30-day extension at a local immigration office. Alternatively, if you are eligible for dual nationality and can obtain your Thai passport during this trip, you could exit Thailand briefly on your UK passport and re-enter on your Thai passport. It's important to note that the rules around using different passports for entering and exiting Thailand may have changed recently, allowing for more flexibility in using both passports rather than strictly one for each entry and exit.