The maths way, (assuming landing) 3 deg. glideslope, call it 6nm away from heathrow.. tan(3)=(X/6) 0.31nm or just over 1900ft. Takeoffs, about 2500ft for a full 747; something like a A320 will be at 4000ft+
http://lhr.webtrak-lochard.com lets you look at past flights.