It's not just the companies immediately around LHR that support and benefit it. There is something called 'the M4 corridor' and international (and UK) companies have deliberately located themselves all along the M4 because of what lies at Junction 4. NO OTHER REASON.
Even our silly govts (plural!) won't allow EGLL to close as it would instantly devalue some 100 miles of business and residential property. Slough, Reading, Braknell, Newbury, Swindon, even Bristol. The M4 corridor is, effectively, an extension of West London. And then add in the M3 that connects Hampshire and all the way to the South Coast?
Yes, we need R3 + T6 but actually, 20 years ago we should have planned to layout not just R3 but R4 as a new cross runway (off set, of course not in the old place) and T7.
However ... I still maintain that we will not see R3 within the next ten years and that EGLL will continue to be marginalised in the world.