Taking the tube is relatively expensive in London compared to Europe. Taking the bus is relatively cheap: £1.50 for any journey, including any number of changes up to 70 minutes after your first hop-on.

It's great that I can commute to work for a £3 round-trip with a change halfway if I choose not to cycle! Of course, that's in South-West London and even then the traffic can get pretty bad. In central it's probably nightmarish to do it regularly.