"I'd add that a major problem is London is the prevalence of terraced houses above high-rises.  You've essentially got the New York Metro area with no apartment towers with comparing."

As a small side point, high rise buildings are a huge upcoming problem. They rely on large furnaces heated by natural gas, and a brief overnight failure of the gas supply in the winter would cause the pipes to freeze, making the buildings uninhabitable. If there is a significant glitch in the energy supply to New York, the city could become vacant in a matter of a week.