Which passport should my Thai wife use when traveling between the UK and Thailand?

UK passport for entering/leaving UK AND checking into flight both going & coming. Thai passport only for entering/leaving Thailand.

We made the mistake once of my wife showing both passports to the airlines; they listed her as a Thai citizen which flagged her upon return to the U.S. since she left the U.S. on her American passport. A discrepancy. American immigration lady yelled at me, suggesting my lack of understanding caused the problem.

*** check-in with Thai passport

*** exit UK with British passport

*** enter Thailand on Thai passport

*** check-in to return flight with UK passport

*** exit Thailand on Thai passport

*** enter UK on British passport

Leave UK on English. Enter Thailand on her Thai passport and then leave on Thai. Enter on English.

in and out of thailand on thai passport,in to UK on UK passport, her indoors been doing it this way for 20 years

English passport? Really?

English passport? lol

Use the Thai passport to enter and leave Thailand. Use the UK passport to leave and enter the UK.

UK passport for entering/exiting the UK, the Thai one for Thailand.

Not rocket science.

At check in for the flight show both passports but leave the UK (immigration) on the British passport and enter Thailand on the Thai passport

If she enters Thailand using her Thai passport then she doesn't have to worry about anything.

If she enters with her UK passport then she'll only receive a 60 day stamp and have to worry about immigration issues.