There are eight different car hire providers operating on Heathrow Airport grounds, with counters in each terminal, where incoming passengers can finalise their order (assuming they booked ahead of time). However, the drop-off locations vary according to the provider.
Each operator has its own dedicated return depot, where drivers can drop their rental car and wait a few minutes while the return procedures are completed. At this point, the operator provides a transfer service to the relevant terminal. Here’s a map provided by the airport detailing each of the car hire firms’ drop-off locations in context of the greater airport grounds.
Be sure to return your vehicle with as much petrol as it had when you set out. If you fail to this, the company will refill the vehicle for you at rates higher than those paid at petrol stations around the airport.