@Stuart ********
In my opinion, the best way for a Thai-UK dual national to leave the UK and enter Thailand is the following:
1) Show only the Thai passport when checking in (unless the ticket to Thailand was purchased using the UK passport information, then use the UK passport so it matches the booking). If they ask to see an onward flight from Thailand within 30 days (possible entering Thailand on a UK passport) then, and only then, should she show her Thai passport.
2) Exit Thailand on the passport of the country she is leaving, namely the UK one.
3) Enter Thailand on the passport of the country she is entering, namely the Thai one. There should be no problem doing this. If there is a problem, and only if there is a problem, she can show her UK passport and explain she is a dual Thai-UK national.
In my opinion, one shows only one passport on each occasion. That is the way you get treated as an unexceptional traveler. Unexceptional travelers move through lines quickly. Only if someone raises a question do you ever need to show the other passport and then only if asked. The dual national traveler knows their situation, there is no need to educate the immigration official or check-in official unless they raise a question.
Don't toss both passports at people and expect them to figure it out for themselves. Know which passport to use and when to use it. Your life will be easier, their life will be easier, and the lives of the people behind you in line will be easier. ;-)