I did this last year. And was successful. Piles of evidence of relationship and how long.you need to declare all your finances. And how you going to support her. Photos of where she will be staying, what she is going to do when here in uk. Be prepared to fly to BKK . She will need to hand in all the paperwork. Biometric scan. Plus finger prints.and hand in her passport,
The thing that clinches it. Is she has a job. And evidence from her employer giving her leave. ( letter)
If successful. You wont know until she get here passport returned.( special delivery) has to be signed for. It took nearly a month. You wont find out until she opens the passport and hopefully stamped up. We spent hours and hours preparing for the application. We used an agent. To double double check everything before we handed it in. They were very good. Was not cheap. Hope this helps. Its getting more and more difficult. Not many succeed. We had 6 months. But unfortunately her company wanted her back in 45 days.