And EI, like all the airlines, doesn't "own" the LHR slots, even though they trade them and count them as assets.
The slot allocators "own" the slots.
Airport Coordination Limited (ACL) is responsible for slot allocation, schedules facilitation and schedule data collection at a large number of varied airports and, in addition, provides a wide range of services to the aviation industry.
Slot allocation is a widely-recognised technique designed to balance the supply and demand for scarce airport capacity, in order to avoid unnecessary congestion and delays, through the issue of permissions for aircraft to land and take off at particular times.