An interesting comment in this article from the Guardian,
History of Lippits Hill base in Waltham Abbey
They placed military cadets, police horses, firearms and dog training facilities at the base, which started to cause residents concern about noise.
The firearms and dog training were eventually moved out under local pressure, but in 1967 three Bell 47G Sioux helicopters were lent to the police by the army for daylight trials over London.
These trials led to similar police helicopters being housed on a permanent basis at Lippitts Hill from 1976.
Today, three quieter £5million choppers with abseiling equipment, Nitesun searchlights, a Skyshout public address system and heat-seaking sensors are based there to help police all over London.
Cllr Syd Stavrou has said the "uneasy truce" between the Met and residents could end when the National Police Air Service (NPAS) takes over the site in April and established no-fly zones end.
She said: “The residents understand the importance of the aircraft in protecting national security but they don’t want the buzzing to disturb them at night or when they are sunbathing in Summer.
“So far there has been an uneasy truce with the base and the Met have been responsive and have had an open dialogue with residents."
FWIW, my Dad was in charge of the Met observers in the 1967 trial; and I was incredibly disappointed (as only a spotty teenager can be) that he wouldn't let the AAC take me up for a ride when they made the offer!