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Perhaps some radical thinking could be done, to relieve the pressure in future on Heathrow, without expansion.
You could say that if BA gets any financial help from the government, it should come with strings attached. No moving Gatwick services to Heathrow, and perhaps some services even move to Gatwick.
If airlines generally reduce Heathrow services for a while, them permanently cancel out some slots.
If and when Virgin disappears, cancel some of their slots from the system.
That way the existing airport could cope better, and the environment would benefit.