Greater London is most often described by using the M25 as a boundary.
(For those who dont know and probably care less - the M25 is the London Orbital Motorway. Its a great big car park which circles London has millions of traffic cones and delusions of being a road where traffic actually moves). I think that Greater London is supposed to have more than 10 million people but our census website died a severe death shortly after they launched it due to too many hits, so your guess is as good as mine.