OK.. There are four holding areas for Heathrow, two to the north and two to the south. The standard procedure for a landing aircraft is to fly a "circuit", which consists of "downwind", where the aircraft flies away from the field, usually parallel to the landing runway, to lose height, next is the "base leg" when the aircraft turns so that it is flying at 90 degrees to the runway heading. Last of all is final approach, when the aircraft is flying straight towards the runway.
Light aircraft can accomplish all this in a mile or two, but larger aircraft have to be properly lined up on final approach from some distance out.
OK... next, consider that major airports are extremely busy so the "circuit" may extend for some miles as individual aircraft follow the one ahead. If it is quiet, the final approach may be only 8 miles long but if it's very busy it may extend to 15-20 miles out.
Major airfield have radar controllers directing the traffic a) to keep it safe and b) to ensure the maximum landing rate to avoid undue delays. The controllers direct the aircraft approximately to the "circuit" described above, although the shape of the circuit may vary somewhat from basics. The two holding areas to the south are Biggin and Ockham. When it is time to start their approach, aircraft are told to leave the holding areas on headings to take them into the circuit. To give controllers maximum flexibility, aircraft from Biggin are usually taken off on a westerly heading whilst those from Ockham leave on roughly an easterly heading. The Biggin aircraft are then turned on to the downwind heading to fit into traffic from Ockham, producing a stream on roughly an easterly heading over the towns of Sutton, Croydon and maybe Bromley before they turn north on the base leg. At the same time, traffic from the holding areas north of Heathrow is being directed similarly on the north side of the field. The two downwind streams are then knitted together as they are turned on to final approach.
There is very much more to it than that, but they are the basics. The radar controllers for Heathrow are located at Swanwick on the south coast and they have control over the whole of the approach sector so they can direct pilots onto various heading to provide separation from other aircraft whilst maintaining the steady landing stream.