What is the best way to obtain a 30-day visa extension in Phuket for a UK citizen?

To obtain a 30-day visa extension in Phuket, you must visit your local immigration office. The best time to apply is when you have 45 days or less remaining on your visa. You'll typically need your passport, a copy of the passport photo page, a copy of your entry stamp into Thailand, a passport-sized photo, and to fill out forms TM30, TM7, STM2, and STM9. These forms can be obtained on-site at the immigration office, and it's advisable to bring copies prepared in advance.