Tourist visa from the UK single entry lasts for 90 days (I’ve just been approved), suspect you arriving on a exemption visa which you can extend by going to the local immigration office for 1900 bhat.
Passport, Copy of passport detail page, copy of arrival stamp on passport original proof of accommodation & copy of accommodation, copy online visa,passport photo not older than 6 month & your 1900 bhat the form you will get at immigration to be completed. I think that’s it I recently did it but got the mrs to do it all 😂