Dr Donna as suggested but it looks like a problem with her website. She's at the Racquet Club on Sukhumvit 49. I used Samitivej Sri Nakarin which is reasonably close to the Airport or at least easily accessible. Keep an eye on the UK Gov website though as there was talk in the news that they are going to scrap the requirement for the preflight tests.
I couldn't find the Line account but I did see an article linking them to Thai Airways so I presume its official. Also saw someone say on a page today they had used them.
I saw that but the advice was a recommendation for a 7 day trip to Thailand. Maybe should have said a 7 day trip to Phuket at the moment as its the only option at this time to apply for a short trip of 7 days unless they open back up the test and go.
If you go through Bangkok at the moment the only option is 7 day quarantine, so a short trip would not be fun unless they open up the test and go again, which is looking unlikely.
I used Antigen on 15th Dec but it can't be a self test as you need certificate to show at check-in. I did mine at Samitivej Srinakarin Hospital in Bangkok. Also check on your airline becsuse some airlines may have their own different rules. I heard Ethiad for example require PCR but UK government are okay with Antigen as shown on UK gov website