Living mindfully in a city requires a different approach to living mindfully within nature. A city like London is actively trying to grab your attention; every headline you see on an abandoned newspaper demands you make a judgement, advertisements shout for your attention and the constant stream of people and traffic is an assault on your senses.
Rather than trying to block these out, mindfulness allows us to absorb them without reacting to them. However, this process requires intense prsenece and a daily meditation practice. The more the world screams for you, the more intensely you need to focus on being here now. When in doubt, come into the body, and become aware of your own consciousness as the central point of all the activity you're experiencing around you. After all, the world doesn't happen to you, it happens in your presence.