Originally Posted by
Phil the Flyer
Presumably the decision to change the direction of runway usage by 180 degrees requires a certain 'lead time' so that, for example, aircraft still holding to the east of LHR can land on 27L/R while, at the same time, ATC can start to hold aircraft to the west of LHR before they can land on 09L/R. If this occurs during a busy period it must give ATC quite a few headaches.
Hence the holding areas are to the North and South of the airport.