Both airports have massively increased the number of flights they operate in a generation, and the number of nighttime aircraft movements. This is justified as planes are quieter now, but
lowering the total noise obviously wasn't the target.
You can the Heathrow noise maps [1].
Kew Gardens [2] is worth visiting, but the current approach to the north runway crosses the garden. In a 500-1000m-wide stretch, it can be difficult to have a conversation since planes are crossing overhead every 45-90 seconds [3].
The local residents may have known about the flight path. The house might be all they could afford, or they thought it wouldn't be so bad, or flight paths might have changed. Whatever the case, we need the 3 million overflown people to continually complain, otherwise there will be nothing pushing back against further expansion of Heathrow.
It is extremely poor planning on successive governments that Heathrow has become Europe's largest airport. London's other airports are to the north or south of nearby larger settlements, so the approach paths fly over far fewer people. Other major airports in Europe have been (or are being) moved to avoid overflying cities.
[1] https://assets.publishing.service.gov.uk/government/uploads/... and https://d3n8a8pro7vhmx.cloudfront.net/southwarkcans/pages/50...
[2] http://www.kew.org/
[3] https://www.flightradar24.com/51.45,-0.32/12