Last question I promise. I know the routine with duel passport holders Thai and British. They with be showing both at check in only. I had no problems before I will be flying them with Qatari airlines. Thanks in advance for advice

19 or 20 is not considered 'unaccompanied' minors, if that's what you are referring to.  Most airlines consider 18 yr olds adults.  Don't see a problem, though you could verify with departing airlines.