Actually, all this is due to noise restrictions. The 09 runways are only used when the wind commands it, but the thing is that there are more settlements on one side than on the other, so it was agreed that if 09 runways are used, 09R will always be used for takeoffs.
For 27 departures, the current rotation system changes twice a day and once a week, I believe. (i.e. ay 1500 and some time in the evening, plus every monday the system changes, eg one week, 27L will be used for departures all mornings, next week it's used all afternoons)
By the way, it is estimated that Heathrow could actually increase its capacity by a few percent (1-3%, I believe) if they were allowed to use the runways for both purposes at the same time, and not use them in the rotation system that is in place now. That is caused by the fact that landings are more efficient than takeoffs, and therefore a bit of capacity is sacrificed. It's a shame about those noise restrictions and agreements with adjacent settlements, huh?