Basically what historyenne and Colleen said--do a couple visits back and forth, get to know each other that way, *then* decide if being over there permanently is something you want to pursue. It can be a lengthy, stressful, and expensive process, and chances are you would need to go down the marriage route, which brings all of the responsibilities and concerns of marriage into play.
I know it's tempting to want to know what your options are up front, because you want to be together, but a lot of things can (and will) change in the next couple of years pertaining to immigration law, fees, etc. It's better to focus on the relationship bit right now, and getting to know each other face-to-face before worrying about all the paperwork and such that you would need. And definitely visit the UK a few times before deciding on anything permanent, just to make sure it's something you could live with, since being married is half of the equation--the other half is seeing if the country is one you can live in, or one you'd be absolutely miserable in.