Me. I tested positive for covid Christmas morning I did a NHS pcr test ten days before I flew tested negative on Friday 25th fit to fly test negative. Arrived in Thailand 28th Feb tested negative just did 2nd test today 5th day tested negative I've not been out since I took first test on day one.
You will need a return flight to Thailand they. Could stop you boarding at your departure airport . Once in Thailand its a simple process to exstend your visa .