When I first moved here, I would often very consciously choose British expressions and words (pavement, worktop, car boot, etc.) simply to avoid confusing my British friends when we spoke, because it made me feel less self conscious to not have to explain Americanisms to confused Brits in every conversation. So, that was deliberate and then it stuck.
I changed all of my spellcheckers to UK English in a fit of paranoia some years back when hunting jobs, afraid that naive employers would mistake my acceptable alternative US spellings for errors and think I was dumb. So, British spelling is pretty natural to me now, too, and I say things like neighbour and flavour without even noticing.
But my accent has also changed markedly since coming here four years ago, and that was entirely unintentional. I hear it a lot, and you know, the first time I caught myself saying "bazzil" instead of "bay-zil" at the grocery store, I really did feel pretty bewildered by it, like I wanted to go home and check to see if I'd lost myself. Who am I today?, I always think. But I've come to accept it.
And in the end, every new person I meet here always asks me where I'm from in the first five minutes of conversation - I still sound very American to British ears. My US people all think I sound like an RAF pilot, to hear them talk, and tease me about my Madonna Accent, which can be a little hurtful because I really don't mean it. But we joke about it and it's fine.
Generally, I don't want to lose touch with my roots and heritage, but I also do my best to accept that hopefully in the end, I will have spent more time in England than I ever did in the states, and in that light, I don't mind becoming British. This is my home, too.