What should a Thai citizen consider when applying for a UK tourist visa?

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

I have just been through this and my girlfriend is in the UK now. She has little or no liquid assets but is the Thai family Matriach and there is a family home, ownership shared with her siblings. I confirmed I was meeting all expenses and submitted copy UK bank statement as evidence of my ability to meet any costs incurred. Also confirmed she would have travel insurance. She will need to go to a VFS office for Bio pic and fingerprints so that could involve some expense. Processing time is 3 weeks from submission and the visa is valid for 6 months - giving six months from date of arrival in UK. Start compliling your data now and read everything you can, using common sense when looking at Visa Agents websites. It can be done without using agents and with her having little funds available but you will need to demonstrate your own good repute as a Sponsor. Her prior travel records will prove useful.

My wife got tourist visa to UK quite easily before we were married. I was on Ship she sorted it all out just followed all the steps as per British embassies website. Took about two weeks

From what you have stated, I think you might have a very difficult time providing sufficient evidence. Would it not make more sense for you to visit her in Thailand until you get married, as it's easy for you. Remember agents cannot help you by bribing someone for this process. So if you are going to go through with it, you need to get her evidence organised and indexed, in the same order immigration ask for it, so that the people making the decision have no excuse to deny her. Good luck.

She must have solid reasons to return to Thailand. Eg. Property , investments, a business, family.. a job alone won't cut it because she could earn more with a job in uk/us/aus etc. Also you've got to show you've got a real and long term relationship.

...I went through all of this chap...glad it's behind me now.

I haven't had a problem, but others have. Note, no matter how long she says she will stay in the UK, the visa is always for 6 months. Do you know what travel history she has had before. Does she own property. Does she have 100,000 in the bank. Does she have children or elderly parents. Who is paying her expenses to and in the UK?