Using Google Earth's ruler tool, I come up with a figure of about 3670 miles. If you took a plane from one to the other, the distance might be pretty different, because the pilot would need to take into account things like the rotation of the Earth, air currents, and weather.
For example, there is a jet stream that flows from west to east across the North Atlantic. A plane flying from Washington DC to London might go out of the way to use the jet stream for a speed boost. A plane flying from London to DC wouldn't want to fly in the jet stream because it's blowing the wrong direction, so it might take a more direct route or even go out of its way to avoid the jet stream.