A health certificate will become mandatory from the 21st March, both for visa applications in London and for pre boarding any flight to Thailand from U.K.
You can’t get an OA visa in Thailand. Must get it from your home country. You will need to check the Italian Thai embassy website for their requirements. They will include compulsory health insurance, financial evidence of either equivalent of
*****
0 baht in an Italian bank or income of equivalent of
*****
baht a month. A criminal record check and a health certificate.
If your landlord regularly rents out your accomadation he may be registered online for TM30. In which case he can (and should) do it (get a screenshot from him of the completed registration). You can do it yourself, however, you will need several documents from the landlord to do so.
if it was in Vietnam then I guess it was the airline asking before you got on flight? That’s normal but is also airline specific. As in some are more efficient at checking than others. When I fly in from UK Qatar airways always checks my visa validity against my stay for example.
The reason you were not in the database, is as you stated, your insurance does not meet the OA insurance criteria. You must buy one of the accepted insurances from the website only. AETNA are there but it’s a different insurance to the one you have sadly (yes it doesn’t make much sense but that’s the rules).