I found that if they have ties to Thailand then it's possible. Such as property in their name, a business or land then it's easier. Best is a finace visa to see if she like the UK. Unlikely but it is possible but be prepared that her family will always be priority before you and any money they make will be returned there
I used an agent after trying so many over a few years. I'm not a retirement visa but wanted a regular input of 65k over a year. Having a bank of Bangkok account keeps it there and the evidence will be easy. Its something about their systems that doesn't add up, including proper visa advice. In all countries the immigration officer will decide but the process should be more straightforward. Maybe money changing hands has something to do with it