How can someone on amnesty in Thailand apply for an unfit to fly certificate to stay after September 26?

Nope.. I apply for buddhist learning visa for 15 months

Smack on Head with Hammer?? Cunning Plan

Try concussion as an excuse

You actually have to be unfit to fly, and one thing they seriously take into consideration, even if you have documentation from a doctor, is that if you can walk in and wait for your queue number to come up, then why would you be unfit to fly by the 26th? If you are recovered enough from whatever, then how much more recovered will you be by the 26th. Did you recently, in the last week, have surgery, or a similar invasive procedure, that will not heal up by then. The most successful medical extensions are being given to people with upcoming surgeries, or bed ridden in the hospital or at home, where the hospital contacts them on your behalf, because you can not travel. It is not easy to get just because you want to stay.

Get a certificate from a hospital and present it at immigration.

Not unfit to fly only residential status to the British Embassy