Send her back to the UK to face the music.
I disagree.
Unless they can find some reason to think it was intentional, it was a mistake. Any American could have made that mistake.
Here she would probably get charged with negligent homicide and probably draw probation.
Their diplomats enjoy diplomatic immunity too.
They do drive on the wrong side of the road.
How about England change their ridiculous street direction. I can see how an American could screw that up.