London offers lots of transportation options. There's the tube, buses, overground trains, car services, taxis and the new rental bike system nicknamed "Boris bikes", but nothing beats walking. You can see so many details you would otherwise miss if you used a faster method of travel.
Did you know that the park area around Buckingham Palace has horse crossing signs? They are much higher than the buttons for human walk signals and I would never have seen them from a vehicle.
Brick Lane has street markets on Sundays. You can't drive down the road. You can only visit the markets on foot.
London has great street art but if you moving too fast, you'll miss the art.
Even in the rain, walking can be so much more enjoyable. It allows you to take your time, travel at a leisurely pace and soak in the atmosphere.