I know many who have had problems and refusals to take their girl to the UK for a visit.
I sailed through in 30 minutes but I'd done some homework on how to ease it through.
We did the following:
1) Money in her bank account
2) Proof of her ownership of land or property
3) Copy of her family documents that show Father, Mother and children
4) Passport
5) My passport
6) Letter from a uk relative assuring accomodation and confirmation that they would ensure her return
7) Applied for a short term visa for 3 months

Proof, confirmed by my landlady in Thai, that we had known each other for longer than 2 years
We turned up , took one look at my passport and checked the database, They then told me to go away. Interviewd my girl for 5 minutes and then called me to the cashier who after payment delivered her passport with a visa stamped in there.
Our biggest hassle was a Heathrow airport where we both got the third degree from a snottie young customs girl. How long have you known each other?, where are you going to live? I was asked 'do you speak Thai?' She was asked' do you speak English?' After both admitting that we only spoke a little of each others language, the retort was 'well how do you communicate?'. At that point I had to refrain myself from the obvious answer!!!
After being cleared I stated 'All this was cleared by embassy staff in BK' The reply was 'so what' . I refrained from using my middle finger for once!!!