Kolibear is quite correct - go aronds would be an issue. Whilst go-arounds are not exactly an emergency situation, the Arrivals controller has to talk to the Deps controller to see where the outbound is going.
They then need to talk to TC Heathrow for a heading and level.
There then needs to be deconfliction with Low Level SVFR traffic and Northolt. All whilst continuing to control traffic streaming in.
As for the size of the zone boundary - you will be amazed at the number of times that older Heavies off LHR only just make the required level of 3000ft by Epsom to the South! (some don't!)