Remember that ADS-B uses line-of sight SSR frequencies so would be of no benefit over the Atlantic, or any remote area.
Indeed, in general ADS-B uses the SSR frequency (1090 MHz), so it is line-of-sight. But that does not mean it has no benefit in remote areas. Although out of sight for ATC, ADS-B can still provide air traffic situation awareness to the flight crew when the aircraft is able to receive ADS-B signals from other aircraft.
For example, this can reduce the separation required for step climbs in oceanic airspace. Instead of the 80+ miles separation needed today to do a step climb, this can be reduced to 10NM using ADS-B, allowing for much more step climbs being made over the Atlantic.
It might have no benefit to the Heatrow director, but to the airlines it does make a difference.