Using a visa agent for the the 🇬🇧 visa application does not guarantee that she will get the visa, 99% will depend on her, so I would recommend that you try your self frist
Your lady will need to confirm that she will be coming back to Thailand ie a job a family a home and most of all the money she has in her bank, same thing for you you will need to confirm that you will be able to fully take care of her in England ie your home and medical and money in your UK bank account