WHBM, I think the answer lies in the concentration of aircraft in the LHR area which will cause pollution. If you have two factories in a 1 square mile radius, the pollution levels will be higher for those living near the factory than 2 factories in a 100 square mile radius...
(Ready to be proved wrong though..)
I think it's sometimes easy to forget how much of a head-start airports like CDG / FRA / AMS have on LHR. Upon landing at AMS recently, I was delighted to see we were 20 mins ahead of schedule. However it took 10-15 minutes to get to the gate, not because of LHR-style traffic jams but because the runway we'd landed on was 3 miles away from the terminal! And there was nothing but countryside and a distinct lack of towns for all of those 3 miles. CDG's location and room to expand is excellent too.