Scheduled flights will operate as normal, nothing is being canceled, however ad-hoc and biz flights that may have expected to operate into LHR will not be doing so. This is not 20% of the daily operation so the story is not accurate or missing some important detail.
Flights out of Heathrow to be cut by a fifth during London 2012 Olympics - Telegraph
Normal scheduled services will continue to operate but unscheduled flights carrying the rich and famous will be banned. Private jets and charter flights, normally able to land at Heathrow, will be told to go to Stansted or other airports outside London
The airport will be allowed a maximum of 36 take-offs and landings an hour, compared with around 43 or 44 during peak hours in summer 2011.
LHR does rather more than 44 takes off and landings per hour on a usual day. Even LGW can get it to 50 an hour. Lazy journos yet again?