Go arounds off 27L do a left hand circuit and rarely seem to climb above 4000' so if any Heathrow traffic sounds particularly noisy over my place (inbounds heading downwind are normally about 6000') I'll bet it will have gone round. So it proved with an Air Bridge 747-8 this afternoon - first time I've seen one other than leaving a vapour trail!
In my old Croydon office which had great views across SW London, I saw something VERY low coming across Merton early one bright and clear morning. It was an Emirates 777 at about 3000'. Think it eventually climbed to 4000' as it's track took it somewhere between Croydon and the TV transmitters at Crystal Palace! Altitudes brought to yyou courtesy FR24!