Just do the application yourself. The most important things to demonstrate are sufficient finances, where she'll stay during the trip and reasons why she will return to Thailand (and leave the UK). A bank statement with a few hundred thousand baht, car ownership docs, property ownership docs and docs for employment/business ownership and you should be fine! You also need a few bits to prove your relationship is legitimate. My gf got hers no problems but it did take 3 months from application.