At heathrow they prefer to use runways 27LR because of noise restrictions and even in light easterly winds will still use these runways. When the wind gets a bit stronger they switch to 9LR. The prevailing winds are however westerly. When 9LR are used though 9L is not used for takeoffs due to noise restrictions, only for landings and 9R for takeoffs. When 27LR are used take offs and landings are switched at 3:00pm every day to give residents some relief.
Runway 5/23 cuts across the over two and as well as being far shorter would also get in the way of other operations, so isn't used.
I, a Brit, am very proud that Heathrow is the busiest international airport in the world, and it shows the skill of ATC and everyone there that they can do this with only 2 runways. Gatwick only has 1 and is still the 8th busiest international airport in the world.