Bananaman

Hi guys and gals, Apologies if this has been asked before (I'm sure it has but I can't find a relevant thread). My Thai wife who resides in the UK with me holds valid British and Thai passports and we plan to visit Thailand next year. When booking flights (Thai Airways), which of my wife's passport details should we provide the airline with and then subsequently check in with? I understand that she has to enter the Kingdom on her Thai passport (unless she wants to be subject to visa rules) but just want to make sure that prior to arrival at Suvarnabhumi we know exactly which passport she needs to be showing at every stage of the journey. There seems to be a lot of conflicting information elsewhere on the net on this topic, so I'm hoping some knowledgable individuals here can settle the matter. Thanks in advance