The offside mirror is useful for gaging your distance from the edge of the macadam. When you start clipping the greenery , you are over far enough.
Get a car with a sunroof, that way you will see something.
I've been over from the states most every year since '93 driving in Scotland, Ireland, Cornwall and around various bits of England. With the exeption of London, which is huge and confusing, and St. Ives, where the streets are UNBELIEVABLY narrow, the cities are easy to get through than you might think because they are much smaller than American cities.
Beware of your first couple of hours. The jet lag makes it very difficult to concentrate. Stop as soon as you can.
Get the smallest car you can use (we travel with 2 people, a harp, guitar, fiddle and luggage and I am 6'-3" so it limits our options) You will get better milage and they fit on the roads better. I have never hired a car in G.B. with more than 1500 miles on the odometer.
I find the sinage very logical, although the word usage is sometimes puzzeling. [ sign enter a divided roadway, our equivalent of an interstate: "Racing by Horse Drawn Vehicles Prohibited" It took me two days to figure out that it probably meant racing PAST horse drawn vehicles.
My most difficult adjustment is when I come home. I didn't make any right turns into the wrong lane while I was there in August but it took me three days back here before I stopped turning into the wrong lane.
Be courteous and every one will assume you are Canadian.