Don't be scared. There's pretty much nothing that can go wrong. The first time I did this trip alone (also BOS-LHR, but on BOAC in 1972) my parents didn't even know there was a UM service and I figured everything out by myself. I did fine and was thrilled to be away from mom and dad and out on my own. The 747s were just coming in, I flew one of the airline's first ones, and I felt like king of the Atlantic back there in 41K. Today the airlines have things a little better organized for kids traveling without adults, but while the flight attendants will certainly know they're there, they won't be closely chaperoned and there are no "boarding school" rules to follow. The kids can do what they want as long as they don't run amok, which I'm sure they won't.

Your kid will remember this trip 50 years from now, so don't freak out about it. She should remember that you gave her the wings to go off alone, not that you were scared about it

PS. If she's taking her mobile phone, which might be a good idea in case of delays / hiccups with her flights, make sure she knows what data services cost when you roam out of the US, or if she's anything like my teenager you might as well be buying the airplane she comes home on.