The Helipad at the Power Station is privately owned but operated by Air Medina - Mal Thwaites formerly of First City Air.
Last I heard it was not available for Commercial operations, Private only. So it may be a viable option if you have a full day parking requirement in London and don't want to line Fayed's pockets! Can't guarantee this info is correct so please contact Mal on:
020 7592 0192