Heathrow, if you go down the A4 out of heathrow towards Slough, and then turn left at a big set of traffic lights, you hit a dual carriageway that goes down the western edge of the airport. Drive down past the new terminal 5 building, round the roundabout and back up, and theres a little turn off there - looks like a dirt track. park up there and walk back down the side of the dual carriageway and you are at the end of one of the runways.
Alternatively, they have the viewing area at h'row, although I'm not sure if you can still take cameras up there - there is a small charge for it though, and I can't remember which terminal it is - probably T3