You can sponsor your Thai girlfriend to visit the UK. as part of the application process they must complete an online visa application form. And book an appointment to submit any supporting documents at the UK visa application center in Thailand.
What would I need to provide?
You would need to show you have adequate funds to cover the proposed visit, and somewhere for the applicant to stay.
What would the applicant need to provide?
The applicant would need to demonstrate that she has a reason to visit the UK, and a reason to return after the proposed visit. They will have a much better chance of obtaining the visa if they have regular employment, assets, for example, land & property, and family ties. These are obviously a reason to return to Thailand after the proposed visit to the UK.
Where to apply for a UK visa in Thailand?
How to apply for a UK visa in Thailand?
Can I marry my girlfriend whilst she is on a tourist visa in the UK?