I don't know why, not sure where from, but I do seem to remember a European ruling not long ago which did give some sort of right along those lines. The UK Govt (or maybe the Irish?) fought it, but from what I remember (though I could easily be horribly wrong, so don't give it any more worth than a suggestion by an occasionally insightful drunk) the fiance is allowed to move because blocking that is effectively interfering with the free movement of the other EU national. It'd have to be on the visa for that country naturally, but it could be done, though I'm not sure if she would have had to be resident in the other EU country for a certain period - the exact thing which is the most probable issue.
It's also quite fun moving to another country and, from a relationship point of view, it'd eliminate some of the problems of living in either the US or UK as you'd both be in the same boat. Equally, depending on where you move, the language barrier is a varying issue. Moving to Holland you can easily live there for a decade and speak no Dutch!
But yes, I wouldn't be coming into this that negatively unless you really don't want to do it, in which case, don't. Coming into it with a set timescale to "give it a chance" is setting yourself up for problems - it'll make you less likely to really give it a go and it might make her feel uncomfortable for a year trying to prove things to you, which can't really be good for the relationship. It'd be like setting an ultimatum and if she's this set, she'll probably be rather unhappy to leave if you didn't like it. Essentially I don't think a 'contract' is really a good way, as it'll just become a vehicle for discussions you'll have to have anyway and if it has any effect will just make things worse.
That said, it isn't that bad here and I think you're worrying too much about the weather - it's a bit depressing, but around London it's not that bad.