Here is the story.
I am on an HSMP visa, and on my next renewal, hope to be Tier 1 (General).
I met a girl. She's American. At the time, she was in London getting a Masters degree. However the university she got said degree from is an American college based in London, so while it is US accredited, it is not UK accredited (so no chance of Tier 1 Student).
We started seeing each other since Sep 2007.
She finished her degree, and then returned to the US, where she is getting her Teaching certification.
We're still seeing each other. Pretty seriously. And its costing a lot in air fare and phone bills!

She finishes her degree in Dec 2009.
I want her to move to the UK in 2010.
So this is where it gets (even more) complicated. Her teaching qualification will be recognised by any UK school. But despite the lack of teachers in the UK, many seem hesitant in sponsoring a work visa. Furthermore, her subject areas do not fall into the Maths & Science categories on the PBS Shortage List (link), so I think this lessens the likelihood of a school sponsoring her Tier 2.
So I thought about trying to get her in as a dependant on my (future) Tier 1 visa, as an unmarried partner. She would live with me, and in the future, we do plan on getting married.
The verbiage on the UK BCA website clearly points out that this relationship is applicable to both heterosexual and homosexual relationships. And there is a requirement that the existing relationship must me "akin to marriage" and have lasted for 2 years prior to the date of application. We actually have been rather serious for a year now, and we would be well past 2 years at the time of application. But my concern is how do we prove that this relationship is "akin to marriage", and I cannot seem to find any solid criteria for defining it.
Any help on this would be much appreciated.