Driving in the UK (for Americans) (Canadians, too)
Having lurked in this forum for months before completing my trip to the UK, I feel I should try to contribute in a meaningful way. I realize that this post is not London specific, but since the London forum appears to receive as much traffic as the other UK forums combined, I'm posting it here.
This is my advice as an American to other Americans who are considering driving themselves around the UK. Differences in driving between the two countries are more than just the "side" of the road that is driven on. These tips were culled from two experiences we had, renting once in Newbury and once in York.
If you're not a confident manual (stick-shift) driver, forget it. Driving on unfamiliar roads with unfamiliar rules is not the time to learn. (The automatics I saw available for rent were expensive luxury cars.)
Rent the smallest car that fits your needs. A Vauxhall Astra was plenty large enough for 2 adults, 2 teens, and luggage.
The "side" you drive on is the smallest difference between the two countries. In the US, right turns are easy. In the UK, left turns are your friend. Be careful and give way (yield) when turning right!
Research the standard road signs (speed, give way, etc.) Research the meaning of the lines for parking, give way (yield), passing, etc. Research the meaning of the large spider web type signs shown as you approach roundabouts. Find out what it means when something is inside ( ). Learn what A and B roads are. Research the speed limit rules and signs.
Research roundabouts, how they work, and what other drivers expect. (The occasional one-lane roundabout at a shopping mall in the US is not sufficient preparation.) There are videos on YouTube and other 'net resources.
Using Google street view, research some of the roads you intend to drive. Notice how the cars are parked, the width of the road and the curves. Notice that there may be walls and hedges very close to the road. Notice that the road may have no shoulders (verges in the UK). Note that directional signs are often located at the exact point of turning, rather than several hundred feet in advance. The width and curviness of the British roads was a bit unnerving for us. (We're from the western US where the roads are wide and straight.)
GPS (sat nav) is a wonderful tool but a map makes it easier to plan your route. You can buy a great AA atlas at many bookstores. Try to plan your route using A and B roads as much as possible. The "white" roads on your map are likely narrow, curvy roads that are best left to the locals who are familiar with them. The sections of motorway we drove on (A6 near Carlisle and A1(M) near Newcastle) were very similar to US freeways with wide lanes and gentle curves.
As you plan your route, make note of the numbers of the roads you'll be traveling (i.e. A59, B6265)--this is much easier than trying to determine whether you need the road to Towne or Upper Towne (which may be in opposite directions). Having at least two people, one to drive and one to navigate and provide lookout is a great help.
Once you've arrived at the car rental place, turn on your GPS to allow it to "find" itself. Before setting off, make sure the GPS has a really good position fix. If not, given the arrangement of roads in the UK, the GPS may think you are not where you actually are. (This happened to us outside the York railway station.) Ask the rental employee for specific directions to where you return the car and the two nearest petrol stations.
Before setting off, take several minutes to become familiar with the controls, particularly the headlights and wipers. Determine whether the car has an Eco anti-idling feature. Ours did and we turned it off just to eliminate another unknown. Figure out how to place the car in reverse. Figure out how to locate 1st and 2nd gears quickly (for roundabouts), without going into reverse. (In one car we drove, reverse was a great distance to the left of 1st. In the other, you entered reverse by first pressing down on the shifter and then moving to the left of 1st.)
Knowing all this greatly eased our driving experience and allowed the driver to relax (partially) and enjoy the beautiful scenery. Hopefully it will help others.