If you have difficulty talking to her, it may help you to write out all your feelings about moving back to the UK. Though, I would leave out the "you owe me" bit.

But, sit down, think about why you think it will be better, why you miss it so much, why it's important to you to give it a go. Tell her all that. It may not convince her to move, but it might open her up to it. It might also open her up to tell you why she has her misgivings, and then they could be addressed.
When people are confronted with ultimatums, they're much less likely to do what you want them to. But if you really express your feelings about it, you might have a better shot at getting what you want.
It also might help for you to do some research about where you might want to live, research about potential jobs, look up houses that you might be able to afford. Having a more concrete idea of what it would be like could also help her.
But you also need to remain open to the idea that she just may not want to do it, and you need to think about what you'll do if that's the case. In any event, you need to really get yourself geared up for a talk, somehow, and you also need to get your immigration status sorted!
