LGW and LHR both have airspace restrictions, you can eye ball LHR inbounds and departures from LGW easily, they're quite close together.
Whilst true to an extent, LGW has the whole of the airspace over Sussex near enough to itself. Heathrow is restricted on all sides, even so far as Bristol to the West, with the majority of transatlantic departures interfering with the majority of Bristol arrivals. I spent two years dirty diving into Bristol having been left 8000ft high by London due to LHR departure traffic.
With Stansted and Luton to the North, City to the East, Biggin to the Southeast, Gatwick to the South, Farnborough and a big chunk of military airspace to the Southwest, there isn't room for an extra 240,000 movements per year without severely limiting these other airports. It's the busiest airspace in the world.