Given that you will be working in Canary Wharf, being on the Jubilee line would be very convenient for commuting. St. John's Wood, and Finchley Road stops along that line (in the northwest of London) would probably fit your bill. St. John's Wood is very popular with American expats (The American School in London is in this neighborhood), and Finchley Road is near Hampstead which is also home to lots of American expats, and has a nice village feel (lots of nice restaurants and shops, and right next to Hampstead Heath--plenty of room to wander!). These are all fantastic areas, but they do come at quite a cost!