If we British really cared about either the environment or fuel burn and maintenance costs we'd see building a third runway at LHR but capping the maximum of flights at say 70% of maximum would both reduce overall congestion and pollution in the UK.
At any time throughout the day, there may be up to 10-20 aircraft circling in the London stacks adding 10-20 minutes extra to their flight times, burning extra fuel, worsening the air quality etc.
Now as a bit of a closet tree hugger, I don't want to see virgin land concreted over nor do I want to see the polar ice caps melt so building a third runway AND capping capacity would would have the effect of reducing the total output of CO2. So this way, everyone is happy - job done - well unless you live in Sipson.