If the area around Heathrow and West London isn't substantial, then where DO you consider "substantial"
Places in a city like residential areas, commercial areas, industrial areas etc. They all sound pretty substantial to me. But a massive private estate, often closed to the public, largely consisting of open fields, gardens, wooded areas and over 1.5km of boggy river banks, easily accessible from the river are hardly "substantial" in my opinion. I suspect a judge may think the same. If it was me going in there again I would probably not inform the campaign.
Also the CAA designed the heli routes through the Heathrow CTR to avoid/minimize over flying congested areas. So there must be lots of "non congested" areas there.