Why don't you take a day trip to EWR, ORD, BOS or DCA and get to know their mass transportation. So when you do go to London you will be more comfortable. DCA would be my first choice as the station is located in the airport. All signs are well marked and you can get to the mall quickly.

Randy is correct. Once you get comfortable with mass transportation you will be loath to rent a car in any big city. Last time I went to Chicago I watched as the El I was in passed miles and miles of backed-traffic. As I got from the airport to downtown in a lot less time then they did.