@Jeremy *******
Agree two separate entities. When checkin in at BKK for returning to UK, all my wife shows is her UK passport as that is what matches the airline licket. Then, the UK passport is put away and only then does she need to show Thai immigration her Thai passport when leaving. But prior to proceeding to passport control when leaving Thailand, she has to show her UK passport and boarding pass to get through the security gate, and only after then show her Thai passport to the immigration officer. (But having said this, we left Thailand last April and there was no immigration officer. Someone scanned mine and my wifes passport but it wasn;t the immigration police in a booth. Maybe thats a new thing?)