ge3....................I know what you're saying, but in your case I think it is best to take the tourist visa approach. This visa, if issued, can easily be extended in the UK. In fact, you won't even need to go to immigration to extend since you can download the form and send everything in the mail.
I'm not a UK citizen but I was living and working legally in the UK for 10 years. Because I wasn't a UK national, my girlfriend was not able to get a fiance visa, even though we had known each other for a few years, and had even gone through the Buddhist marriage ceremony. We applied for a tourist visa, which was denied twice, but grant on the third application. She got six months, but we simply extended this in the UK and she ended up staying there for close on three years. We also considered getting married while in the UK, and we determined that we could do so if we had wanted to, despite the fact that she only had a tourist visa. BTW........we've been married for 12 years now.
My friend on the other hand (UK citizen) applied for a fiance visa for his lady, and the amount of red tape was unbelievable. The even wanted the measurements of the rooms in his home in the UK. His lady had a very good job in Thailand, and was in fact only wanting to visit the UK for two weeks. In the end, he also went the tourist visa route.
Another friend of mine, also from the UK has been married to a Thai women for the last three to four years, and she still has not managed to get into the UK based on marriage. Having said that, they have never taken my advice and applied in person. Instead, they have used agents three times already, and as far as I'm aware, it's already cost him close to 100K (baht) BTW.....the agents they've used also "guarantee" success.
So, my advice is...................you and/or your girlfriend should apply in person, and in my opinion, you should apply for the tourist visa first. Your girlfriend and you should also rehearse your story/plans. For example, she only wants to visit for x amount of weeks, during which time she'll be staying with you, and etc, etc, etc.
They will want to interview her for the visa, so the more fluent her English is, the better. If you are in Thailand at the time, you can also asked to be interviewed, which they may or may not allow. When I was going through this with my girlfriend, I was here in Thailand for the third application, and at my request, I was called into the interview room after they had spoken to her. I explained our situation, and the visa was granted. However, I had been over to Thailand dozens of times to see my lady, and we had thousands of photos to prove our relationship was for real. In our case, the biggest problem was that neither of us were from the UK.
A long reply, but maybe it can help in some way. Best of luck.

Don't try to impress me with your manner of dress cos a monkey himself is a monkey no less - cold fact