Your Thai girlfriend gets her passport back from the British embassy with the long awaited tourist visa inside. The visa will allow her to enter the UK for up to six months. The only obstacle in your way now is the air ticket right? Wrong. Just because she has a visa, your Thai girlfriend getting into the UK will come across one more important hurdle

Your Thai girlfriend will have to go through UK immigration

When arriving in the UK your Thai girlfriend will have to go through UK immigration before being allowed to enter the UK. Even with a valid visa in her passport, she still can be refused entry. Ever year many Thai partners with valid UK entry visas never get past the airport and are sent back to Thailand on the next available flight.

When your Thai girlfriend gets to the immigration desk she will be asked several questions by the immigration office on the desk. If the officer is not happy with any of her answers, she will be taken to an interview room and asked more questions. The visa officer is the last line of defense against people entering the UK, he or she has every right to refuse entry to any person with a valid UK visa.
If you Thai girlfriend gets refused entry into the UK, she will then be left waiting in the airport for the next available flight, this can be the same day or even could take days to get her a seat on a plane. When she arrives back in Thailand she will be arrested and detained until the price of the air ticket is paid in full.

Even with a valid visa your Thai girlfriend could still be refused entry into the UK as has to get past the immigration officer at the airport on arrival in the UK. If the answers are not to the visa officers liking you will be on one side of the airport while your girlfriend will be on the other waiting a flight home.