Yeh I find it hard to believe too, but the equivalent of 7.6 shards are planned to be demolished. Sure there is no 'false scarcity' being 'centrally planned' into the system so that rents and values always magically go up.
http://www.newstatesman.com/jonn-elledge/2014/05/believe-it-or-not-london-doesnt-have-enough-offices