The active rw should be the thing where the AI planes take off and land. They choose their stuff depending on the winds.
As a test, load your plane at EGLL, set the weather manually and watch the guys changing the active runway. If this doesn't work, something else may be wrong.

Current METAR 2011/10/03 23:50 EGLL 032350Z 25009KT 9999 BKN011 17/14 Q1019.
So if you would use rw weather right now, you should see rw27 operations in the sim unless they aim for 8 kts of tailwind on the 09 ones, which I doubt.

There would be a way of limiting the departures and arrivals to certain runways in FSX, but I don't think that the scenery you spoke of has those limits implemented. This wouldn't make sense.
The real Heathrow has a sort of rotation in place, so they actually use some runways even with tailwind components up to a certain level (I would have to look up the values though) for noise abatement reasons and stuff.