susanpmtbc, welcome to the Boards! I have a few comments to add to the others regarding my own experiences, as I rented a Car on my last trip to England. I'm from Canada, so had the same "mental adjustment" to make in terms of driving on the "other" side of the road.
Some suggestions and comments for you to consider:
> It's best to arrange the car rental at your home travel agent, as they can usually get some fairly good deals with the rental companies (ie: a weekly rate).
> Check with your credit card company regarding the CDW (Collision Damage Waiver) or other insurance. Some 'premium' credit cards will take care of the collision and comprehensive, IF you book the car rental charges on that card. Usually the renter still has to pay for public liability. Make sure you understand exactly what portion of the insurance your credit card covers, and what part it doesn't cover!
> I haven't checked a Map with respect to the places you want to visit, but I would suggest renting the car at a location somewhere
outside the city of London. I was told by several knowledgeable people NOT to drive in London and I heeded that advice! Not only is it very "busy", but you'll get nicked with the congestion charge as well. Stansted is probably a good location.
> If this is your first attempt at driving on the "other" side of the road, I would strongly suggest an automatic transmission! I found that to be extremely helpful, as there's enough to concentrate on without having to worry about shifting as well. I've got more confidence now, and could probably handle a manual transmission on my next trip.
> Rent a car that is big enough to handle four people (I believe that's how many will be in your group?). When the rental company advertises that a Car is large enough for "two people with luggage", that's an accurate measure and you WON'T be able to use that for three or more people! A mid size (perhaps something like a Renault Laguna?) would be good, BUT I would be cautious with a full sized Car. If you're used to driving a "land yacht" in the U.S. with wide roads, you'll find it far different in England!!! The roads in many parts of the country (except for the Motorways) can be quite narrow, and it's a bit unsettling to pass a huge Lorry (truck) on a narrow two lane road! it requires concentration all the time!
> Watch your speed limit! You don't want to get a ticket in the mail a month after you get home! The rental company will make sure this is passed on to you (it's in the contract, I believe). From what I remember, most of the Motorways are 60 MPH (can't remember if these were shown in KPH?). I don't have a problem with either speed measurement, as we use KPH in Canada.
> Be very careful about Parking, especially in towns! If you can find a pay lot, that's a good method BUT keep track of your time as you don't want to find a "Boot" on the car when you come back from shopping. Overtime parking charges can be VERY expensive! Some locations use the small cardboard clock, which you leave on the top of the dashboard. Ask the rental company about this when you rent the car.
> Generally speaking, I found driving
easier in the north of England (North York area) than in the south. Driving in Winchester was VERY challenging, due to the one-way streets. The volume of traffic also seemed more intense in the south, and be wary of the ring road near Gatwick (can't remember the number?) as it's been described as the "world's largest parking lot".
> Be careful with
right hand turns, as these are handled in the opposite way that you do at home. Also, in many places in North America, one can make a right hand turn on a red light, after ensuring it is safe to do so. This is NOT the case in England apparently. Drivers can NOT make a left hand turn on a red light, but must wait until the light turns green. Incidentally, the British traffic lights sequence to "stop" is green, then amber, then red; the reverse is then red, to amber to green. I kind of like that extra amber light, as one is able to prepare to start moving again. I wish the traffic lights here in North America were set up that way!
> Do some study on the net regarding the "protocol" or "rules" (whatever you want to call them?) for roundabouts. They're not actually that difficult, but I found that it helps to have a good Map at hand, so that you have some idea which direction or town you're heading towards. The rental firm will provide you with a basic Map, but you might want to consider something better, such as a Michelin Map. I got lost a couple of times as I didn't have a proper Map, but thank goodness I had a Cell Phone which got me going in the right direction again. You might find it helpful to read
www.travelfurther.net/dictionaries/driving.htm.
> The Driver of a motor vehicle is NOT allowed to talk on a Cell Phone while driving. Not sure if this is something you are concerned about, but I was very careful in this regard when I was driving in England.
> Don't forget on the Motorway, the left hand lane is the "slow" lane. British drivers seem to move quite quickly, and they will soon let you know if you're impeding the flow of traffic!
> Don't be too shocked at the price of Fuel / Petrol. I was paying the equivalent of about Cdn$ 2.15 / liter at the time I was in England. At that time, we were paying about Cdn$ 0.90 / liter at home. There's no point in worring about the cost, that's just the way it is. If one wants the convenience of a Car, that's the cost of convenience.
> Finally
(and perhaps most importantly) when you first get into the Car at the Airport, spend 15 minutes or so to become familiar with the controls and the general layout. You'll probably find (as I did) that some things are different than they are in North America. The computerized Key and the "start" button were a bit of surprise! When you're on a narrow two lane road with lots of traffic, it's not the best time to be trying to figure out where the headlights or the wiper controls are!!!
When I first pulled out of the lot at Manchester Airport, I was quite nervous and felt quite awkward. However, after about 15-20 minutes I started to feel quite comfortable with it. I have friends from the U.K. that live in my community, and they told me that it wouldn't be too difficult to make the adjustment, and they were right. They also said that I wouldn't find it difficult adjusting back to driving in Canada, and they were right about that also. When I started driving out of the lot of my local airport, it felt like I'd never been away. Considering that I had been driving on the 'other' side of the road for several weeks, it was rather a strange feeling to say the least!
I found that having a Car on my trip was extremely valuable and useful!!! It allowed me to get to some locations that aren't well served by public transport, and allowed me to use my time MUCH more efficiently. I only had a couple of weeks to travel, so didn't want to waste any time at all! I plan on renting a Car again on my next trip to the U.K. (hopefully next year), but will NOT use one in London as it would be expensive and unnecessary, as public transport is so good in the city. I will probably rent in Bath, as I will be travelling in the Cotswolds and around Salisbury plain (most likely - haven't worked out my Itinerary yet - I also want to get back to York, as it was one of my favourite spots!).
Hope this has been some help. Good luck and happy travels in England!!!
begins!! We'll be staying in Wolverhampton with our daughter, who does not have a car. I'm trying to figure out how to get to Wolves from the airport and finding that it's not a simple task! 
We get in at around 7:30 p.m. It looks like we can take a train into London and then another one to Wolves...but I've been wondering about the feasibility of hiring (renting) a car. We'll be there for 10 days and plan on doing some traveling, of course. I've been thinking that this might be the most economical and practical way to go, but my husband (and 20 year old son!) aren't so sure of this! Does anyone have experience with driving in England, where everything will be the opposite of what we're used to here in the States?
begins!! We'll be staying in Wolverhampton with our daughter, who does not have a car. I'm trying to figure out how to get to Wolves from the airport and finding that it's not a simple task! 
We get in at around 7:30 p.m. It looks like we can take a train into London and then another one to Wolves...but I've been wondering about the feasibility of hiring (renting) a car. We'll be there for 10 days and plan on doing some traveling, of course. I've been thinking that this might be the most economical and practical way to go, but my husband (and 20 year old son!) aren't so sure of this! Does anyone have experience with driving in England, where everything will be the opposite of what we're used to here in the States?
Driving in England is no hassle - drivers are as manic as any other place! - but your point about being the opposite to what you're used to and to Europe is valid (damn that Napoleon!). I've driven in countries with both left and right hand drive and it does take extra concentration (esp roundabouts - yikes!) but it's not impossible. A few points to consider:
.
. 
. No signs are in metric at all. 