Pre-departure COVID test is not required for the destination country (London Heathrow) and neither is health insurance covering US$20,000. However, it's pretty obvious health insurance covering said amount is required even for transit passengers spending less than 24 hrs at Suvarnabhumi. I raised the question because there's a conflicting information between Thailand Pass FAQs PDF issued by foreign ministry (link posted by ubonjoe) and Civil Aviation Authority of Thailand announcement (link below) that does not mention pre-flight COVID test as requirement (dated April 7th, 2022). The planned flight (Tokyo, Narita → BKK → HRW) is 2 months away and there's a good chance it will all be scrapped if Thailand Pass is abolished by then. However, with all the variables it's hard to book the flight beforehand while the TG sales lasts. As you said, I should probably ask the airline. https://www.caat.or.th/wp-content/uploads/2022/04/guidelines-Inter-flight-with-Transit-Transfer-passegers-TH-ENG.pdf