Looking at a satellite map to the East of Heathrow Airport, there is a small passenger jet parked in a walled off area. Looking at the aerodrome chart the entire area is listed as Bealine Base, which I assume is used for BA operations. What it the purpose of this walled off area?
It allows them to do a run-up test of the engines without blowing debris at other parked aircraft, people, or things on the ground.
[Credit to Ralph J]
The walls have a structure that allows the noise of engine run-ups to be absorbed as well. Engine run-ups, especially multi-engine run ups, can be very loud.