Op I am a little older than you and have come across this before on more than one occasion with others, and have witnessed some very poor treatment of thoroughly decent people.
You op are a lovely person, a kind and thoughtful friend and willing to be a great friend, but you are not her 'best friend' or even a good friend. You were perhaps great work friends at one point, but she has long since moved on and you have not. I mean this very kindly, because you have invested so much into this friendship, she on the other hand has invested very very little, and therefore thinks nothing of going away with her 'uni' friends and leaving you behind, she doesn't believe you will bail on her because you are a nice person, she believes you will swallow the hurt and get on with doing your job, mainly organising her wedding and pandering to her generally.
She sees herself as superior to you.
She sees her life as superior to yours, and although you are welcome enough you are never going to be invited into her 'inner circle' that is reserved for people like 'them'.
For whatever reason, and I suspect it may be lined to class and money actually, as you describe your dp as northern, left leaning and a teacher and if she is surrounded by millionaires they almost certainly do not share his passion for equality. You are seen as different, not one of them. That is why you are not there.
The minute you pull out of MOH expect this to be a drama where you are painted as the monster for 'ruining' her wedding.
Your friendship is dead, not because of the trip or the wedding, but because she treats you as if you are second class, less than her, and it took this trip to shine a light on this fact, but my guess is that the facts have long been there.
You are too good a friend for someone like her, she will use and abuse all of her friends, and will end up with nothing genuine in her life.
Personally I would choose to end this with dignity. A simple text.
' I would be grateful if you could find an alternative to MOH, as I feel I can not commit any longer. I know you will plenty of friends to ask, so I don't think it should cause much of problem. Wishing you the best with the planning of your wedding, and the day itself'
End of, that way she can't blame you for letting her down, and you will have behaved with dignity and grace. She is never going to admit to treating you badly, so there is no point in having it out with her. She does not value your friendship enough op.