If you are living anywhere central, meaning within the bounds of the North and South circular roads (roughly tube zones 1, 2 and 3), public transport is by far the cheapest and easiest way to travel, and often the fastest. Anywhere will be within 10-15 minutes of a rail station except for odd spots like Crouch End. Bicycles may be better depending on where you are and are going, but cars are only worth it, on average, when you have to carry goods, are going to the countryside, or late at night when the roads are clear. The rest of the time, traffic jams, congestion charges, finding a parking spot, parking fees, etc, make them an unattractive option. In other words - cycle or use public transport, and rent a car/taxi when you need one, which won't be that often.