Several years ago I spent about 6 weeks touring England, Scotland and Wales. I got used to driving from the wrong side of the car on the wrong side of the road :) But the most dangerous thing I found was as a pedestrian. On this side of the pond, its natural to look to the left as you step off a curb. Over there, the cars in the near lane are coming from the right. One other difference is the use of the horn. In GB if you hear a horn, something bad is about to happen. Over here, we also use them for doorbells and saying 'Hi'. Sometimes, just for fun. But you never know.
One more thing. If you're a fan of Haggis, you're out of luck. What you put in haggis, we throw in the garbage.