Well I cant tell you all our story as it was so long ago, 25years ,that I have forgotten most of it, but the one advantage you do have is that your gf is here now. Briefly, my wife was on a student visa and we met while I was working in her college in London. I was separated and had another 2 years to wait until I could apply for divorce, then another 6 months for the absolute. By the way, a new relationship was the last thing on my mind.
The long and short of it was her visa finished, some court appearances to extend and eventually appeal against her being thrown out of the country failed , and, though we were living together for at least a year it got to within 3 weeks of us having to move to the P.I. Lunar House completely ignored all our evidence of being a totally above board couple in an established relationship.
Someone suggested our local MP, but I didn`t hold out much hope as he was the very famous right wing, and right hand man to Maggie Thatcher. We went to see him,explained the situation, he wrote a letter and within the week my wife was granted indefinite leave to remain. The man remains my hero to this day.
So my advice is, fight to keep your gf/fiancee here,
and take all the proof with you and ask your MP for assistance.
Or for an easier time my advice is dump her and marry an English girl