This year April i had to write letter showing 6 Months wages proving i can support her take pic of house utility bill my partner works government hospital as nurse so don't know if this was reason why
Just get her to go British embassy get visa fill out form and you can write letter saying that you are going to support her in England embassy will tell her what you need to prepare
you should be ok when she go to British embassy in Bangkok they should let her know all what she needs from you to send to her via email best of luck and enjoy your time together
You need to write letter saying your going to sponsor her and give your address plus last 6 Months payslip plus proof of your address this is what I had to provide when my partner came in April but she is government hospital worker and has a 10 year old son don't know if this makes sense