Heathrow Director may come along and explain it better, but all departures are designed so traffic procedurally ie non radar remains below arriving traffic. On a DVR departure for instance, traffic crosses below arrivals routing to OCK VOR which normally descends to FL70 depending on the altimeter setting. The 250 kt speed limit is normally cancelled by the departures controller as early as possible and the aircraft is climbed above the SID altitude as soon as it is clear of the arrival pattern.
It's even more complicated with Heathrow landing on 09L as BCN departures are vectored parallel to arrivals from OCK and BIG and climbed above the arrivals by the Heathrow Director so you get two tracks of aircraft parallel, one descending and the other climbing.