You only need to ask a visa question in UK House, you will get the right answer! I guarantee it. Your GF can ask on your behalf even. I don't usually put my head on the block for anyone easily, but some of the helpers on there went through the processes, saw all the changes on a daily basis, read all good and horror stories. They know possibly as much as a visa consultant. I have so much free time this year, I try to kill time by doing something useful; I have been using loads of forums from hobby to taxes, I know a good forum such as this when I see one. people here are so friendly and very helpful. As a nosey Parker, I have been asked to post questions on ladyinter.com on some very shy Thai ladies' behalves, most are horrid and tricky; reading UKBA website does not help much, the helpers gave me all the correct answers.
We just helped out a Thai lady who went to the UK on a visitor's visa then got persuaded to get married to her Brit BF. They possibly have saved thousands by asking if they could change the type of visa to a settlement one so the lady does not have to leave the UK. The husband has three little kids; he was hoping if all failed, they would go the whole way to the EU Courts on grounds of Human Rights. They cannot change the visa type, so will lose the fees. Going all the way is going to cost possibly five to ten times more than simply coming back to Thailand to get a Spouse visa which should be granted within 3-4 months.
Just beautify your bank balance, it needs to be good a a few more years. She should keep her job if she has one. Having her child in Thailand could be a good incentive for her to not overstay her visa in the UK, in UKBA's view.
I don't see many UK visa questions on here. LOL I guess people ask the kind of questions in the 'expats in the UK' forum, but no worries. You need all the help you can get. I am usually pretty good at doing a research, on this horrid subject of visa; it seems similar to the UK laws, you need to look into a lot of cases. Sharing experiences are the best help. That is why I am here, gathering info for my hubby to live a happy life in Thailand. I know it is weird for a Thai to be in an 'expats in Thailand' forum. You are just doing the same in the reverse. LOL
Good luck.
Excuse my typos and other errors, I am very sleepy. Thanks!