This might help:
"A 'westerly preference' has been operated at Heathrow since 1962 as a noise mitigation measure. The preference provides for 'westerly operations' (i.e. when landing aircraft approach Heathrow from the east over London and take off to the west over Berkshire) to continue when there is a light easterly following wind up to 5-knots, if the runways are dry and any cross-wind does not exceed 12-knots. Maintaining westerly operations in this way reduces the need for aircraft to take off in an easterly direction over Hounslow, Ealing, Twickenham etc., where the departure routes cannot be designed to avoid the most densely populated areas; in contrast to the position west of the airport where the departure routes pass between the main built-up areas.
Westerly preference is an important noise mitigation measure during the daytime. Aircraft taking off use more power than when landing and consequently make more noise. As most aircraft also take off at a steeper gradient than the 3° final approach glideslope followed by landing aircraft, take-off noise footprints are usually larger than landing ones, so their noise affects more people. However, the quid pro quo for reducing take-offs over west London is that landing aircraft have to make their final approach over London more often, and these are concentrated on the two final approach tracks."
Westerly preference doesn't operate at night, by the way.