No problem switching from US to British driving. You are so aware and alert to the difference that you are on top of it. Coming back to the US after an extended time in Britain is a different story. You relax because you are back to what you are used to- but you aren't used to it anymore.
Stepping off of a curb in Britain is dangerous because you tend to look the wrong way. You may encounter slow moving farm tractors on express roads in England. When you are watching traffic to enter a round about, a bicyclist in the round about will not be where you are looking.
Driving in Europe ( My experience is primarily Germany) isn't dangerous because the rules are so different, its because they are so much the same except for a few critical differences. You get complacent because it is so familiar - except for what isn't.
Examples:
Absolute right of way from the right instead of right has right of way if you arrive at the same time.
You are responsible to make sure that you aren't being passed when turning left at an intersection.
Also the speed differences on the autobahn, Nearly everyone on an American interstate is driving withing 10 mph of everyone else. On the autobahn, the speed ranges from about 50 mph with some trucks to unbelievably fast.
I love the right of way signs - the opposite of stop and yield signs and used on the streets protected by stop and yield signs.
