The British embassy does not issue the letter for unable to travel due to Covid. I know because they emailed me 2 weeks ago when I asked for one. They also said in the email that immigration know that the British embassy no longer issues this letter. I went back to immigration with this information and they told me I need a plane ticket out of Thailand and a reason why I can’t leave now. So I just flew to Phanom Phen and back the next day. Stamped in no problem on 30 day visa exempt.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.