A permanência prolongada na Tailândia pode ter diferentes sanções, dependendo de quanto tempo alguém passou do país.
Even if you overstay for just one day, legal actions could be taken up against your person. Usually though when you overstay for one day, it doesn’t carry a fine with it. The fine of 500 baht per day of overstay only begins after the second day, so if your stay expired on the 15th of March and you only reported to the immigration by the 20th, your fine will only be 4days of overstay or 2,000 baht.
Violators can pay the fine at the immigration office, or at the airport. The process won’t take 10 minutes, and fines could be even paid in advance too. The fine for overstaying should not exceed 20,000 baht in the max. If you can’t avoid overstaying though, it’s a good thing to voluntarily clear your name by showing yourself up in the immigration. If you do this, you will avoid imprisonment and further implications.
Deixo um link para o Thaiembassy onde você encontrará informações sobre multas e outras informações úteis:
Embaixada da Tailândia - Siam Legal
Seria bom entrar em contato com o consulado / embaixada britânica por telefone e perguntar como proceder e, então, tomar a decisão sobre qual opção é a melhor.