If she's going to the UK for a holiday you, as her sponsor, just have to demonstrate that you can pay for her trip and accommodate her in the UK.
More problematic will be showing that she has a genuine reason to return to Thailand after the holiday, as she doesn't have a job. Do you have a life together in Thailand? If you can show that you live together in Thailand and are just having a holiday in the UK it would help. Do you have a work permit/retirement extension for Thailand?
There are plenty of agents available (including a sponsor of this Forum) but without a good reason for her to return to Thailand, I'm not sure that they can do much.