The most energy efficient form of vehicular road transport is coach.

All forms of public transport use less energy per person km/mile than the average car. In general taxis are slightly more energy efficient than cars, but London Black Cabs are significantly more fuel hungry. 

London buses have increased use of hybrid fuel engines for greater efficiency. 

London underground, light rail and tram and overground rail are all much more energy efficient than car travel.

Source: UK Department of Environment Food and Rural Affairs (Defra)