The British Embassy no longer issues visa extension letters.
On Monday 2nd November 2020, the British Embassy Bangkok received verbal confirmation from Thai Immigration Bureau that Embassy letters are not required to extend visas within Thailand. Instead, you can complete an affidavit at your local Immigration Office.
If you want to remain in Thailand after your current visa expires, or you have questions about your immigration status in Thailand you should speak to your local Immigration Office or visit the Thai Immigration website.